Investments - Realized Investment Gains and Losses (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	$ 51	$ 67	$ 97	$ 35
Less income taxes (benefits)	17	23	32	12
Net realized investment gains (losses)	34	44	65	23
Fixed Maturities [Member]
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	14	6	16	(12)
Equity securities [Member]
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	1	11	34	1
Commercial Mortgage Loans [Member]
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	(1)	4	(1)	4
Other Investments Including Derivatives [Member]
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	$ 37	$ 46	$ 48	$ 42