Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated other comprehensive income, net of tax, beginning of period	$ 1,780		$ 1,693
Accumulated other comprehensive income, net of tax, end of period	1,780
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income Loss Before Tax [Abstract]
Accumulated other comprehensive income, before tax, beginning of period	2,625	$ 2,094
Other comprehensive income before reclassification, before tax	290	351
Reclassification adjustment, before tax	(159)	(61)
Effect on other comprehensive income, before tax	131	290
Accumulated other comprehensive income, before tax, end of period	2,756	2,384
Accumulated Other Comprehensive Income (Loss), Tax [Abstract]
Accumulated other comprehensive income, tax, beginning of period	908	722
Other comprehensive income before reclassification, tax	102	121
Reclassification adjustment, tax	(56)	(21)
Effect on other comprehensive income, tax	46	100
Accumulated other comprehensive income, tax, end of period	954	822
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated other comprehensive income, net of tax, beginning of period	1,802	1,562	1,717	$ 1,372
Other comprehensive income before reclassification, net of tax	188	230
Reclassification adjustment, net of tax	(103)	(40)
Effect on other comprehensive income, net of tax	85	190
Accumulated other comprehensive income, net of tax, end of period	1,802	1,562
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income Loss Before Tax [Abstract]
Accumulated other comprehensive income, before tax, beginning of period	(26)	(42)
Other comprehensive income before reclassification, before tax	0	0
Reclassification adjustment, before tax	1	1
Effect on other comprehensive income, before tax	1	1
Accumulated other comprehensive income, before tax, end of period	(25)	(41)
Accumulated Other Comprehensive Income (Loss), Tax [Abstract]
Accumulated other comprehensive income, tax, beginning of period	(8)	(14)
Other comprehensive income before reclassification, tax	0	0
Reclassification adjustment, tax	0	1
Effect on other comprehensive income, tax	0	1
Accumulated other comprehensive income, tax, end of period	(8)	(13)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated other comprehensive income, net of tax, beginning of period	(17)	(28)	(18)	(28)
Other comprehensive income before reclassification, net of tax	0	0
Reclassification adjustment, net of tax	1	0
Effect on other comprehensive income, net of tax	1	0
Accumulated other comprehensive income, net of tax, end of period	(17)	(28)
Accumulated Net Unrealized Gain (Loss) on Deferred Costs, Reserves And Other
Accumulated Other Comprehensive Income Loss Before Tax [Abstract]
Accumulated other comprehensive income, before tax, beginning of period	(9)	1
Other comprehensive income before reclassification, before tax	3	(4)
Reclassification adjustment, before tax	(1)	0
Effect on other comprehensive income, before tax	2	(4)
Accumulated other comprehensive income, before tax, end of period	(7)	(3)
Accumulated Other Comprehensive Income (Loss), Tax [Abstract]
Accumulated other comprehensive income, tax, beginning of period	(3)	1
Other comprehensive income before reclassification, tax	2	(1)
Reclassification adjustment, tax	(1)	0
Effect on other comprehensive income, tax	1	(1)
Accumulated other comprehensive income, tax, end of period	(2)	0
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated other comprehensive income, net of tax, beginning of period	(5)	(3)	(6)	0
Other comprehensive income before reclassification, net of tax	1	(3)
Reclassification adjustment, net of tax	0	0
Effect on other comprehensive income, net of tax	1	(3)
Accumulated other comprehensive income, net of tax, end of period	(5)	(3)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income Loss Before Tax [Abstract]
Accumulated other comprehensive income, before tax, beginning of period	2,590	2,053
Change in unrealized gains, net, on investments available for sale, before tax	131	290
Change in pension obligations, before tax	1	1
Change in life deferred acquisition costs, life policy reserves and other, before tax	2	(4)
Effect on other comprehensive income, before tax	134	287
Accumulated other comprehensive income, before tax, end of period	2,724	2,340
Accumulated Other Comprehensive Income (Loss), Tax [Abstract]
Accumulated other comprehensive income, tax, beginning of period	897	709
Change in unrealized gains, net, on investments available for sale, tax	46	100
Change in pension obligations, tax	0	1
Change in life deferred acquisition costs, life policy reserves and other, tax	1	(1)
Effect on other comprehensive income, tax	47	100
Accumulated other comprehensive income, tax, end of period	944	809
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated other comprehensive income, net of tax, beginning of period	1,780	1,531	$ 1,693	$ 1,344
Change in unrealized gains, net, on investments available for sale, net of tax	85	190
Change in pension obligations, net of tax	1	0
Change in life deferred acquisition costs, life policy reserves and other, net of tax	1	(3)
Effect on other comprehensive income, net of tax	87	187
Accumulated other comprehensive income, net of tax, end of period	$ 1,780	$ 1,531