Investments in debt and equity securities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Debt and Equity Securities
Cost Basis	$ 1,317	$ 1,577
Unrealized pretax net gains (losses)	50	58
Fair Value	1,367	1,635
U.S. treasury bonds
Schedule of Debt and Equity Securities
Cost Basis	9	9
Unrealized pretax net gains (losses)	0	0
Fair Value	9	9
Other U.S. and non-U.S. government bonds
Schedule of Debt and Equity Securities
Cost Basis	60	71
Unrealized pretax net gains (losses)	0	1
Fair Value	60	72
Corporate bonds
Schedule of Debt and Equity Securities
Cost Basis	489	701
Unrealized pretax net gains (losses)	3	7
Fair Value	492	708
Asset-backed securities
Schedule of Debt and Equity Securities
Cost Basis	90	129
Unrealized pretax net gains (losses)	0	0
Fair Value	90	129
U.S. governmental agency mortgage-backed securities
Schedule of Debt and Equity Securities
Cost Basis	225	291
Unrealized pretax net gains (losses)	(2)	1
Fair Value	223	292
Residential
Schedule of Debt and Equity Securities
Cost Basis	10	12
Unrealized pretax net gains (losses)	0	0
Fair Value	10	12
Commercial
Schedule of Debt and Equity Securities
Cost Basis	36	59
Unrealized pretax net gains (losses)	0	2
Fair Value	36	61
Large capitalization value
Schedule of Debt and Equity Securities
Cost Basis	280	243
Unrealized pretax net gains (losses)	32	30
Fair Value	312	273
REIT
Schedule of Debt and Equity Securities
Cost Basis	77	25
Unrealized pretax net gains (losses)	2	0
Fair Value	79	25
Smaller company growth
Schedule of Debt and Equity Securities
Cost Basis	41	37
Unrealized pretax net gains (losses)	15	17
Fair Value	$ 56	$ 54