Investments in debt and equity securities (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Cost Basis	$ 144
Due after one year through five years, Cost Basis	418
Due after five years through ten years, Cost Basis	121
Due after ten years, Cost Basis	21
Due in one year or less, Fair Value	144
Due after one year through five years, Fair Value	417
Due after five years through ten years, Fair Value	120
Due after ten years, Fair Value	22
Cost Basis	994
Fair Value	989
Schedule of Debt and Equity Securities
Cost Basis	1,425	$ 1,317
Fair Value	1,439	1,367
Available-for-sale Securities, Proceeds, Gains and Losses
Proceeds from the sale of available-for-sale securities	930	694	$ 351
Gross gains from the sale of available-for-sale securities	109	55	64
Gross losses from the sale of available-for-sale securities	5	4	$ 2
U.S. governmental agency mortgage-backed securities
Schedule of Debt and Equity Securities
Cost Basis	265	225
Fair Value	261	223
Residential
Schedule of Debt and Equity Securities
Cost Basis	8	10
Fair Value	8	10
Commercial
Schedule of Debt and Equity Securities
Cost Basis	17	36
Fair Value	$ 17	$ 36