Investments in Debt and Equity Securities (Details 3) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Cost Basis	$ 160		$ 160
Due after one year through five years, Cost Basis	540		540
Due after five through ten years, Cost Basis	122		122
Due after ten years, Cost Basis	17		17
Due in one year or less, Fair Value	159		159
Due after one year through five years, Fair Value	531		531
Due after five years through ten years, Fair Value	120		120
Due after ten years, Fair Value	18		18
Cost Basis	1,161		1,161
Fair Value	1,139		1,139
Schedule of Investments in Debt and Equity Securities
Cost Basis	1,161		1,161		$ 994
Available-for-sale securities, fair value	1,139		1,139		989
Net unrealized gain (losses) for equity securities with a readily determinable fair value	10		14
Realized net gains (losses) recognized on the sale of equity securities with a readily determinable fair value	4		4
Available-for-sale Securities, Proceeds, Gains and Losses
Proceeds from the sale of available-for-sale securities	41	$ 244	181	$ 431
Gross gains from the sale of available-for-sale securities	0	38	0	40
Gross losses from the sale of available-for-sale securities	0	$ 1	0	$ 3
U.S. governmental agency mortgage-backed securities
Schedule of Investments in Debt and Equity Securities
Cost Basis	300		300		265
Available-for-sale securities, fair value	290		290		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	15		15		17
Available-for-sale securities, fair value	$ 14		$ 14		$ 17