Investments in Debt and Equity Securities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Investments in Debt and Equity Securities
Cost Basis		$ 431
Unrealized Pretax Net Gains (Losses)		19
Fair value		450
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	47	42
Unrealized Pretax Net Gains (Losses)	0	0
Fair value	47	42
Corporate bonds
Schedule of Investments in Debt and Equity Securities
Cost Basis	677	585
Unrealized Pretax Net Gains (Losses)	(12)	(1)
Fair value	665	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	304	265
Unrealized Pretax Net Gains (Losses)	(8)	(4)
Fair value	296	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	16	17
Unrealized Pretax Net Gains (Losses)	(1)	0
Fair value	15	17
Total debt securities
Schedule of Investments in Debt and Equity Securities
Cost Basis	1,121	994
Unrealized Pretax Net Gains (Losses)	(21)	(5)
Fair value	$ 1,100	989
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