Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	$ (1,382)
Shareholders' other comprehensive income (loss), net of tax	300	$ (132)	$ (314)
Accumulated other comprehensive income (loss), ending	(1,082)	(1,382)
Securities [ Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	362	418	620
Other comprehensive income (loss) before reclassifications, pre-tax	35	(48)	(389)
Other comprehensive income (loss) before reclassifications, tax (expense) benefit	(19)	6	157
Other comprehensive income (loss) before reclassifications, after-tax	16	(42)	(232)
Reclassification adjustment, pre-tax	(77)	(22)	46
Reclassification adjustment, tax (expense) benefit	27	8	(16)
Reclassification adjustment, after-tax	(50)	(14)	30
Shareholders' other comprehensive income (loss), net of tax	(34)	(56)	(202)
Accumulated other comprehensive income (loss), ending	328	362	418
Derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	3	7	(8)
Other comprehensive income (loss) before reclassifications, pre-tax	(1)	0	10
Other comprehensive income (loss) before reclassifications, tax (expense) benefit	0	0	(3)
Other comprehensive income (loss) before reclassifications, after-tax	(1)	0	7
Reclassification adjustment, tax (expense) benefit	1	2	(4)
Reclassification adjustment, after-tax	(2)	(4)	8
Shareholders' other comprehensive income (loss), net of tax	(3)	(4)	15
Accumulated other comprehensive income (loss), ending	0	3	7
Derivatives [Member] | Other Operating Expenses [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification adjustment, pre-tax	1	1	12
Derivatives [Member] | Net Realized Investment Gains [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification adjustment, pre-tax	(4)	(7)	0
Translation of foreign currencies [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(369)	(274)	(62)
Other comprehensive income (loss) before reclassifications, pre-tax	309	(95)	(224)
Other comprehensive income (loss) before reclassifications, tax (expense) benefit	(5)	0	12
Shareholders' other comprehensive income (loss), net of tax	304	(95)	(212)
Accumulated other comprehensive income (loss), ending	(65)	(369)	(274)
Postretirement benefits liability [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	(1,378)	(1,401)	(1,486)
Other comprehensive income (loss) before reclassifications, pre-tax	(22)	(29)	63
Other comprehensive income (loss) before reclassifications, tax (expense) benefit	8	10	(23)
Other comprehensive income (loss) before reclassifications, after-tax	(14)	(19)	40
Reclassification adjustment, tax (expense) benefit	(24)	(22)	(23)
Reclassification adjustment, after-tax	47	42	45
Shareholders' other comprehensive income (loss), net of tax	33	23	85
Accumulated other comprehensive income (loss), ending	(1,345)	(1,378)	(1,401)
Reclassification adjustment for amortization of net losses from past experience and prior service costs [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification adjustment, pre-tax	64	64	68
Reclassification adjustment for settlement losses [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification adjustment, pre-tax	$ 7	$ 0	$ 0