Investments in Debt and Equity Securities (Details 3) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Cost Basis	$ 127
Due after one year through five years, Cost Basis	513
Due after five through ten years, Cost Basis	117
Due after ten years, Cost Basis	19
Due in one year or less, Fair Value	127
Due after one year through five years, Fair Value	506
Due after five years through ten years, Fair Value	115
Due after ten years, Fair Value	19
Cost Basis	1,093
Fair Value	1,076
Schedule of Investments in Debt and Equity Securities
Cost Basis			$ 431
Available-for-sale securities, fair value			450
Net unrealized losses for equity securities with a readily determinable fair value	2
Realized net gains (losses) recognized on the sale of equity securities with a readily determinable fair value	0
Available-for-sale Securities, Proceeds, Gains and Losses
Proceeds from the sale of available-for-sale securities	73	$ 89
Gross gains from the sale of available-for-sale securities	0	1
Gross losses from the sale of available-for-sale securities	0	$ 1
U.S. governmental agency mortgage-backed securities
Schedule of Investments in Debt and Equity Securities
Cost Basis	292		265
Available-for-sale securities, fair value	285		261
Residential
Schedule of Investments in Debt and Equity Securities
Cost Basis	8		8
Available-for-sale securities, fair value	8		8
Commercial
Schedule of Investments in Debt and Equity Securities
Cost Basis	17		17
Available-for-sale securities, fair value	$ 16		$ 17