How to map column and entity propery of different datatypes in entity framework code first

I am using Entity Framework 5 - Code first.

I have a database that I am connecting to that already exists for some time now (I did not create it). There is a table called T_Customers. It contains a list of all the customers. It has the following structure (only partially shown):

Customer_id | numeric (5, 0) | auto increment | not null (not set as primary key)
FName | varchar(50) | not null
LName | varchar(50) | not null

My Customer class:

public class Customer : IEntity
{
     public int Id { get; set; }

     public string FirstName { get; set; }

     public string LastName { get; set; }
}

My IEntity interface:

public interface IEntity
{
     int Id { get; set; }
}

I have the following in my database context class:

public class DatabaseContext : DbContext
{
     public DatabaseContext(string connectionString)
          : base(connectionString)
     {
     }

     public DbSet<Customer> Customers { get; set; }

     protected override void OnModelCreating(DbModelBuilder modelBuilder)
     {
          modelBuilder.Configurations.Add(new CustomerConfiguration());
     }

     public new DbSet<TEntity> Set<TEntity>()
          where TEntity : class, IEntity
     {
          return base.Set<TEntity>();
     }
}

My customer configuration class:

class CustomerConfiguration : EntityTypeConfiguration<Customer>
{
     internal CustomerConfiguration()
     {
          this.ToTable("T_Customers");
          this.Property(x => x.Id).HasColumnName("Customer_id");
          this.Property(x => x.FirstName).HasColumnName("FName");
          this.Property(x => x.LastName).HasColumnName("LName");
     }
}

I'm trying to be consistent in my entity declarations, I need all the IDs to be integers. This customer ID is of type num