Investments (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Net Investment Income [Line Items]
Derivative, notional amount	$ 163	$ 167
Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments	1,338	1,411
Derivative liability, fair value, gross asset	1
Derivative liability, fair value, gross liability		$ (3)
Future capital call commitments for limited partnership investments	428
Mortgage loan commitments	47
Commitments to purchase or fund privately placed debt securities	198
Commitments to sell various privately placed debt securities	162
Common Stock
Net Investment Income [Line Items]
Net investment income	$ (1)