WCF dataContracts and LINQ to SQL

Iam trying to create a basic WCF service library web service which connects to a database via LINQ to SQL and returns list of DataContract marked objects but I keep recieving this error:

Error    1    Cannot implicitly convert type 'System.Linq.IQueryable' to 'System.Collections.Generic.List'. An explicit conversion exists (are you missing a cast?)    C:\Users\Cameron\Documents\Visual Studio 2008\Projects\CwkServer\CwkServer\CwkService.cs    34    22    CwkServer

I feel its just a basic error on my part but cant see it. Here is my code

cwkService
       public List getDetails()
       {
           List details = from list in db.Listings
                     select new getCarDetails
                     {
                         CarName = list.CarType.CarTypeName,
                         NumDoors = list.CarType.NumberOfDoors,
                     };
           return details;
}

ICwkService
  [DataContract]
    public class getCarDetails
    {
        string carName;
        int numDoors;

        [DataMember]
        public string CarName
        {
            get { return carName; }
            set { carName = value; }
        }

        [DataMember]
        public int NumDoors
        {
            get { return numDoors; }
            set { numDoors = value; }
        }
    }

Any advice would be greatly appreciated, kind of hit a brick wall with it now

Thanks
Jason