Investments in debt and equity securities (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Cost Basis	$ 139
Due after one year through five years, Cost Basis	647
Due after five years through ten years, Cost Basis	46
Due after ten years, Cost Basis	17
Due in one year or less, Fair Value	139
Due after one year through five years, Fair Value	635
Due after five years through ten years, Fair Value	43
Due after ten years, Fair Value	17
Cost Basis	1,171
Fair Value	1,151
Schedule of Debt and Equity Securities
Cost Basis	1,171	$ 994
Fair Value	1,151	989
Available-for-sale Securities, Proceeds, Gains and Losses
Proceeds from the sale of available-for-sale securities	247	930	$ 694
Gross gains from the sale of available-for-sale securities	0	109	55
Gross losses from the sale of available-for-sale securities	0	5	$ 4
Unrealized gain (loss) on investment in equity securities held on report date	(25)
U.S. governmental agency mortgage-backed securities
Schedule of Debt and Equity Securities
Cost Basis	301	265
Fair Value	297	261
Residential
Schedule of Debt and Equity Securities
Cost Basis	7	8
Fair Value	7	8
Commercial
Schedule of Debt and Equity Securities
Cost Basis	14	17
Fair Value	$ 13	$ 17