Investments in debt and equity securities (Details 3) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Cost Basis	$ 188
Due after one year through five years, Cost Basis	421
Due after five years through ten years, Cost Basis	13
Due after ten years, Cost Basis	26
Due in one year or less, Fair Value	189
Due after one year through five years, Fair Value	423
Due after five years through ten years, Fair Value	13
Due after ten years, Fair Value	26
Cost Basis	919
Fair Value	920
Schedule of Debt and Equity Securities
Cost Basis	1,317	$ 1,577
Fair Value	1,367	1,635
Available-for-sale Securities, Proceeds, Gains and Losses
Proceeds from the sale of available-for-sale securities	694	351	$ 434
Gross gains from the sale of available-for-sale securities	55	64	38
Gross losses from the sale of available-for-sale securities	4	2	$ 2
U.S. governmental agency mortgage-backed securities
Schedule of Debt and Equity Securities
Cost Basis	225	291
Fair Value	223	292
Residential
Schedule of Debt and Equity Securities
Cost Basis	10	12
Fair Value	10	12
Commercial
Schedule of Debt and Equity Securities
Cost Basis	36	59
Fair Value	$ 36	$ 61