Investments - Realized Investment Gains and Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	$ 117	$ 75	$ 214	$ 110
Less income taxes (benefits)	42	27	74	39
Net realized investment gains (losses)	75	48	140	71
Fixed Maturities [Member]
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	20	7	36	(5)
Equity securities [Member]
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	6	(1)	40	0
Commercial Mortgage Loans [Member]
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	0	0	(1)	4
Other Investments Including Derivatives [Member]
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	$ 91	$ 69	$ 139	$ 111