Investments in debt and equity securities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Debt and Equity Securities
Cost Basis	$ 1,171	$ 994
Unrealized pretax net gains (losses)	(20)	(5)
Fair Value	(1,151)	(989)
U.S. treasury bonds
Schedule of Debt and Equity Securities
Cost Basis	9	10
Unrealized pretax net gains (losses)	0	0
Fair Value	(9)	(10)
Other U.S. and non-U.S. government bonds
Schedule of Debt and Equity Securities
Cost Basis	42	42
Unrealized pretax net gains (losses)	0	0
Fair Value	(42)	(42)
Corporate bonds
Schedule of Debt and Equity Securities
Cost Basis	735	585
Unrealized pretax net gains (losses)	(15)	(1)
Fair Value	(720)	(584)
Asset-backed securities
Schedule of Debt and Equity Securities
Cost Basis	63	67
Unrealized pretax net gains (losses)	0	0
Fair Value	(63)	(67)
U.S. governmental agency mortgage-backed securities
Schedule of Debt and Equity Securities
Cost Basis	301	265
Unrealized pretax net gains (losses)	(4)	(4)
Fair Value	(297)	(261)
Residential
Schedule of Debt and Equity Securities
Cost Basis	7	8
Unrealized pretax net gains (losses)	0	0
Fair Value	(7)	(8)
Commercial
Schedule of Debt and Equity Securities
Cost Basis	14	17
Unrealized pretax net gains (losses)	(1)	0
Fair Value	$ (13)	(17)
Large capitalization value
Schedule of Debt and Equity Securities
Cost Basis		287
Unrealized pretax net gains (losses)		(3)
Fair Value		(284)
REIT
Schedule of Debt and Equity Securities
Cost Basis		104
Unrealized pretax net gains (losses)		6
Fair Value		(110)
Smaller company growth
Schedule of Debt and Equity Securities
Cost Basis		40
Unrealized pretax net gains (losses)		16
Fair Value		(56)
Equities
Schedule of Debt and Equity Securities
Cost Basis		431
Unrealized pretax net gains (losses)		19
Fair Value		$ (450)