Segment Information (Reconciliation of Net Investment Income from Segments to Consolidated Net Income (Loss)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Abstract]
Total net investment income	$ 4,990	$ 4,874	$ 4,827
Annuities Segment [Member]
Net Investment Income [Abstract]
Total net investment income	1,038	1,033	1,004
Retirement Plan Services Segment [Member]
Net Investment Income [Abstract]
Total net investment income	899	859	846
Life Insurance Segment [Member]
Net Investment Income [Abstract]
Total net investment income	2,643	2,562	2,541
Group Protection Segment [Member]
Net Investment Income [Abstract]
Total net investment income	168	176	184
Other Operations [Member]
Net Investment Income [Abstract]
Total net investment income	$ 242	$ 244	$ 252