Investments in Debt and Equity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Investments in Debt and Equity Securities
Cost Basis	$ 1,161	$ 994
Unrealized Pretax Net Gains (Losses)	(22)	(5)
Fair value	1,139	989
U.S. treasury bonds
Schedule of Investments in 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 Investments in Debt and Equity Securities
Cost Basis	48	42
Unrealized Pretax Net Gains (Losses)	0	0
Fair value	48	42
Corporate bonds
Schedule of Investments in Debt and Equity Securities
Cost Basis	721	585
Unrealized Pretax Net Gains (Losses)	(11)	(1)
Fair value	710	584
Asset-backed securities
Schedule of Investments in Debt and Equity Securities
Cost Basis	61	67
Unrealized Pretax Net Gains (Losses)	0	0
Fair value	61	67
U.S. governmental agency mortgage-backed securities
Schedule of Investments in Debt and Equity Securities
Cost Basis	300	265
Unrealized Pretax Net Gains (Losses)	(10)	(4)
Fair value	290	261
Residential
Schedule of Investments in Debt and Equity Securities
Cost Basis	7	8
Unrealized Pretax Net Gains (Losses)	0	0
Fair value	7	8
Commercial
Schedule of Investments in Debt and Equity Securities
Cost Basis	15	17
Unrealized Pretax Net Gains (Losses)	(1)	0
Fair value	$ 14	17
Equity securities
Schedule of Investments in Debt and Equity Securities
Cost Basis		431
Unrealized Pretax Net Gains (Losses)		19
Fair value		450
Large capitalization value
Schedule of Investments in Debt and Equity Securities
Cost Basis		287
Unrealized Pretax Net Gains (Losses)		(3)
Fair value		284
Real estate investment trust (REIT)
Schedule of Investments in Debt and Equity Securities
Cost Basis		104
Unrealized Pretax Net Gains (Losses)		6
Fair value		110
Smaller company growth
Schedule of Investments in Debt and Equity Securities
Cost Basis		40
Unrealized Pretax Net Gains (Losses)		16
Fair value		$ 56