Investments in Debt and Equity Securities (Details) - USD ($) $ in Millions	Mar. 31, 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	$ 13	10
Unrealized Pretax Net Gains (Losses)	0	0
Fair value	13	10
Other U.S. and non-U.S. government bonds
Schedule of Investments in Debt and Equity Securities
Cost Basis	51	42
Unrealized Pretax Net Gains (Losses)	0	0
Fair value	51	42
Corporate bonds
Schedule of Investments in Debt and Equity Securities
Cost Basis	649	585
Unrealized Pretax Net Gains (Losses)	(9)	(1)
Fair value	640	584
Asset-backed securities
Schedule of Investments in 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 Investments in Debt and Equity Securities
Cost Basis	292	265
Unrealized Pretax Net Gains (Losses)	(7)	(4)
Fair value	285	261
Residential
Schedule of Investments in Debt and Equity Securities
Cost Basis	8	8
Unrealized Pretax Net Gains (Losses)	0	0
Fair value	8	8
Commercial
Schedule of Investments in Debt and Equity Securities
Cost Basis	17	17
Unrealized Pretax Net Gains (Losses)	(1)	0
Fair value	16	17
Total debt securities
Schedule of Investments in Debt and Equity Securities
Cost Basis	1,093	994
Unrealized Pretax Net Gains (Losses)	(17)	(5)
Fair value	$ 1,076	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