session becomes null in home controller mvc 4

Question

I am new to mvc. My question is: After entering the user credentials when user logsIn then a HttpPost controller is called and that controller calls a method for login inside the model class, inside that class a session is created which stores user details.

Now the problem starts, i can access the session in HttpPost login controller but not inside the Home controller.

Controller Code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using Test.Models;
using System.Web.Security;
using System.Configuration;
using System.Xml;

namespace Test.Controllers
{
public class LoginController : Controller
{        
    #region LogIn

    //
    // GET: /Login/
    [HttpGet]
    public ActionResult Index()
    {
    }

    [HttpPost]
    public ActionResult Index(LogInModel model, string Command)
    {
    //Check the button clicked
        switch (Command)
        {
            case "Log In":
                if (ModelState.IsValid)
                {
                    string sUser = model.CheckValidity();

                    if (sUser != "")
                    {                            
                        //Check session value
                        string strCheckSession =  Session["UserDetail"].ToString();

                        return RedirectToAction("Index", "Home");                            
                    }
                    else
                    {
                        ModelState.AddModelError("", "Invalid UserName / Password!");
                        return View(model);
                    }
                }
                break;
            case "Forgot Password":
                return RedirectToAction("Index", "ForgotPassword");                    
                break;
            default:
                break;
        }
        // If we got this far, something failed, redisplay form

        return View(model);
    }


    #endregion
}

}

Model Code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Xml;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel;
using System.Globalization;
using System.Web.Security;
//using Helpers;
using System.Web.Mvc;
using System.Diagnostics;
using System.IO;
using System.Security;
using System.Security.Cryptography;
using System.Text;
using System.Web.UI;

namespace Test.Models
{
public class LogInModel //: Page
{
    [Required(ErrorMessage = "User name required.")]
    [DisplayName("User name")]
    public string UserName { get; set; }

    [Required(ErrorMessage = "Password required.")]
    [DataType(DataType.Password)]
    [DisplayName("Password")]
    public string Password { get; set; }

    [DisplayName("Remember me?")]
    public bool RememberMe { get; set; }

    /// <summary>
    /// Checks if user with given password exists in the database
    /// </summary>
    /// <param name="_username">User name</param>
    /// <param name="_password">User password</param>
    /// <returns>True if user exist and password is correct</returns>
    public string CheckValidity()
    {
        try{
        XmlDocument objIxml = new XmlDocument();
        //add input details in objIxml
        XmlDocument objOxml = new XmlDocument();            
        //The Authentication method will connect to database and return the output as xml
        objOxml = Authentication(objIxml);
        HttpContext.Current.Session["UserDetail"] = objOxml.OuterXml;
          if(objOxml.OuterXml != "")
            return "Valid user";
          else
            return "";
        }
        catch (Exception ex)
        {
            //throw Exception;
        }            
    }
}

Home View:

@{
    ViewBag.Title = "Home";
    Layout = "~/Views/Shared/TestMaster.cshtml";
 }

 @{ 
    var sessionVar = Session["UserDetail"].ToString();
    <span>@sessionVar</span>
 }
 </div>

Home Controller code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using Test.Models;

namespace Test.Controllers
{
     public class HomeController : Controller
     {
     //
     // GET: /Home/

    public ActionResult Index()
    {
        //object reference Error in session                        
        string str = Session["UserDetail"].ToString();            
        return View();
    }

  }
}

Now in Session["UserDetail"].ToString() it says object reference error !