Fair Value Disclosure of Financial Instruments (Textual) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fixed Rate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Notes payable	$ 2,610	$ 2,500
Notes payable at fair value	2,710	2,540
Variable Rate Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Notes payable	820	1,020
Notes payable at fair value	$ 820	$ 970