Investments in Debt and Equity Securities (Details 3) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Cost Basis	$ 135		$ 135
Due after one year through five years, Cost Basis	518		518
Due after five through ten years, Cost Basis	123		123
Due after ten years, Cost Basis	18		18
Due in one year or less, Fair Value	135		135
Due after one year through five years, Fair Value	509		509
Due after five years through ten years, Fair Value	120		120
Due after ten years, Fair Value	18		18
Cost Basis	1,121		1,121
Fair Value	1,100		1,100
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	6		4
Realized net gains (losses) recognized on the sale of equity securities with a readily determinable fair value	0		0
Available-for-sale Securities, Proceeds, Gains and Losses
Proceeds from the sale of available-for-sale securities	67	$ 98	140	$ 187
Gross gains from the sale of available-for-sale securities	0	1	0	2
Gross losses from the sale of available-for-sale securities	0	$ 1	0	$ 2
U.S. governmental agency mortgage-backed securities
Schedule of Investments in Debt and Equity Securities
Cost Basis	304		304		265
Available-for-sale securities, fair value	296		296		261
Residential
Schedule of Investments in Debt and Equity Securities
Cost Basis	7		7		8
Available-for-sale securities, fair value	7		7		8
Commercial
Schedule of Investments in Debt and Equity Securities
Cost Basis	16		16		17
Available-for-sale securities, fair value	$ 15		$ 15		$ 17