Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pre-tax
Other comprehensive (loss) income	$ (150)	$ 458	$ (841)	$ 794
Tax
Other comprehensive (loss) income	32	(159)	177	(276)
After-tax
Other comprehensive (loss) income	(118)	299	(664)	518
Unrealized net holding gains and losses arising during the period, net of related offsets
Pre-tax
Unrealized net holding during the period	(249)	479	(990)	901
Less: reclassification adjustment	(80)	64	(106)	174
Other comprehensive (loss) income	(169)	415	(884)	727
Tax
Unrealized net holding during the period	53	(167)	208	(315)
Less: reclassification adjustment	17	(22)	22	(61)
Other comprehensive (loss) income	36	(145)	186	(254)
After-tax
Unrealized net holding during the period	(196)	312	(782)	586
Less: reclassification adjustment	(63)	42	(84)	113
Other comprehensive (loss) income	(133)	270	(698)	473
Unrealized foreign currency translation adjustments
Pre-tax
Other comprehensive (loss) income	(9)	17	(14)	12
Tax
Other comprehensive (loss) income	2	(6)	3	(4)
After-tax
Other comprehensive (loss) income	(7)	11	(11)	8
Unrecognized pension and other postretirement benefit cost
Pre-tax
Unrealized net holding during the period	1	(3)	4	(3)
Less: reclassification adjustment	(27)	(29)	(53)	(58)
Other comprehensive (loss) income	28	26	57	55
Tax
Unrealized net holding during the period	0	2	(1)	2
Less: reclassification adjustment	6	10	11	20
Other comprehensive (loss) income	(6)	(8)	(12)	(18)
After-tax
Unrealized net holding during the period	1	(1)	3	(1)
Less: reclassification adjustment	(21)	(19)	(42)	(38)
Other comprehensive (loss) income	$ 22	$ 18	$ 45	$ 37

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