Investments in Debt and Equity Securities (Details 3) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Cost Basis	$ 169		$ 169
Due after one year through five years, Cost Basis	415		415
Due after five through ten years, Cost Basis	55		55
Due after ten years, Cost Basis	24		24
Due in one year or less, Fair Value	170		170
Due after one year through five years, Fair Value	417		417
Due after five years through ten years, Fair Value	55		55
Due after ten years, Fair Value	24		24
Cost Basis	933		933
Fair Value	934		934
Schedule of Investments in Debt and Equity Securities
Cost Basis	1,393		1,393		$ 1,317
Available-for-sale securities, Fair Value	1,457		1,457		1,367
Available-for-sale Securities, Proceeds, Gains and Losses
Proceeds from the sale of available-for-sale securities	244	$ 109	431	$ 304
Gross gains from the sale of available-for-sale securities	38	10	40	43
Gross losses from the sale of available-for-sale securities	1	$ 1	3	$ 3
U.S. governmental agency mortgage-backed securities
Schedule of Investments in Debt and Equity Securities
Cost Basis	245		245		225
Available-for-sale securities, Fair Value	243		243		223
Residential
Schedule of Investments in Debt and Equity Securities
Cost Basis	8		8		10
Available-for-sale securities, Fair Value	8		8		10
Commercial
Schedule of Investments in Debt and Equity Securities
Cost Basis	17		17		36
Available-for-sale securities, Fair Value	$ 17		$ 17		$ 36