Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pre-tax
Other comprehensive (loss) income	$ (691)	$ 336
Tax
Other comprehensive (loss) income	145	(117)
After-tax
Other comprehensive (loss) income	(546)	219
Unrealized net holding gains and losses arising during the period, net of related offsets
Pre-tax
Unrealized net holding during the period	(740)	444
Less: reclassification adjustment	(25)	132
Other comprehensive (loss) income	(715)	312
Tax
Unrealized net holding during the period	155	(155)
Less: reclassification adjustment	5	(46)
Other comprehensive (loss) income	150	(109)
After-tax
Unrealized net holding during the period	(585)	289
Less: reclassification adjustment	(20)	86
Other comprehensive (loss) income	(565)	203
Unrealized foreign currency translation adjustments
Pre-tax
Other comprehensive (loss) income	(5)	(5)
Tax
Other comprehensive (loss) income	1	2
After-tax
Other comprehensive (loss) income	(4)	(3)
Unrecognized pension and other postretirement benefit cost
Pre-tax
Unrealized net holding during the period	3	0
Less: reclassification adjustment	(26)	(29)
Other comprehensive (loss) income	29	29
Tax
Unrealized net holding during the period	(1)	0
Less: reclassification adjustment	5	10
Other comprehensive (loss) income	(6)	(10)
After-tax
Unrealized net holding during the period	2	0
Less: reclassification adjustment	(21)	(19)
Other comprehensive (loss) income	$ 23	$ 19