Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pre-tax
Other comprehensive income (loss)	$ 1,033	$ 518	$ (2,027)
After-tax
Other comprehensive income (loss)	673	339	(1,316)
Tax
Other comprehensive income (loss)	(360)	(179)	711
Unrealized net capital gains and losses
Pre-tax
Unrealized net holding during the period	866	486	(1,896)
Less: reclassification adjustment	374	(180)	112
Other comprehensive income (loss)	492	666	(2,008)
After-tax
Unrealized net holding during the period	562	316	(1,233)
Less: reclassification adjustment	243	(117)	73
Other comprehensive income (loss)	319	433	(1,306)
Tax
Unrealized net holding during the period	(304)	(170)	663
Less: reclassification adjustment	(131)	63	(39)
Other comprehensive income (loss)	(173)	(233)	702
Unrealized foreign currency translation adjustments
Pre-tax
Other comprehensive income (loss)	72	15	(89)
After-tax
Other comprehensive income (loss)	47	10	(58)
Tax
Other comprehensive income (loss)	(25)	(5)	31
Unrecognized pension and other postretirement benefit cost
Pre-tax
Unrealized net holding during the period	232	(263)	(64)
Less: reclassification adjustment	(237)	(100)	(134)
Other comprehensive income (loss)	469	(163)	70
After-tax
Unrealized net holding during the period	153	(169)	(39)
Less: reclassification adjustment	(154)	(65)	(87)
Other comprehensive income (loss)	307	(104)	48
Tax
Unrealized net holding during the period	(79)	94	25
Less: reclassification adjustment	83	35	47
Other comprehensive income (loss)	$ (162)	$ 59	$ (22)