Investments in Debt and Equity Securities (Details 3) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Cost Basis	$ 209		$ 209
Due after one year through five years, Cost Basis	622		622
Due after five through ten years, Cost Basis	50		50
Due after ten years, Cost Basis	27		27
Due in one year or less, Fair Value	210		210
Due after one year through five years, Fair Value	634		634
Due after five years through ten years, Fair Value	52		52
Due after ten years, Fair Value	27		27
Cost Basis	1,262		1,262
Fair Value	1,282		1,282
Schedule of Investments in Debt and Equity Securities
Cost Basis	1,658		1,658		$ 1,577
Available-for-sale securities, Fair Value	1,710		1,710		1,635
Available-for-sale Securities, Proceeds, Gains and Losses
Proceeds from the sale of available-for-sale securities	109	$ 110	304	$ 238
Gross gains from the sale of available-for-sale securities	10	32	43	38
Gross losses from the sale of available-for-sale securities	1	$ 1	3	$ 2
U.S. governmental agency mortgage-backed securities
Schedule of Investments in Debt and Equity Securities
Cost Basis	282		282		291
Available-for-sale securities, Fair Value	286		286		292
Residential
Schedule of Investments in Debt and Equity Securities
Cost Basis	10		10		12
Available-for-sale securities, Fair Value	10		10		12
Commercial
Schedule of Investments in Debt and Equity Securities
Cost Basis	62		62		59
Available-for-sale securities, Fair Value	$ 63		$ 63		$ 61