Investments - Realized Investment Gains and Losses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	$ 237	$ 169	$ 57
Less income taxes (benefits)	81	60	17
Net realized investment gains (losses)	156	109	40
Fixed Maturities [Member]
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	25	23	(82)
Equity securities [Member]
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	52	(1)	36
Commercial Mortgage Loans [Member]
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	(1)	4	(2)
Other Investments Including Derivatives [Member]
Realized Gains (Losses) on Investments [Line Items]
Realized investment gains (losses) before income taxes	$ 161	$ 143	$ 105