Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning			$ (1,382)
Shareholders' other comprehensive income (loss), net of tax	$ 22	$ 129	258	$ 535
Accumulated other comprehensive income (loss), ending	(1,124)		(1,124)
Realized investment (gains) losses	(117)	(75)	(214)	(110)
Other operating expenses	2,696	2,428	7,497	7,038
Net unrealized appreciation on securities [ Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	660	1,135	542	612
Other comprehensive income (loss) before reclassifications, pre-tax	(5)	94	163	606
Reclassification adjustment, pre-tax	(26)	(6)	(76)	5
Other comprehensive income (loss), pre-tax	(31)	88	87	611
Accumulated other comprehensive income (loss), before tax, ending	629	1,223	629	1,223
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(224)	(367)	(180)	(194)
Other comprehensive income (loss), securities, before reclassifications, tax (expense) benefit	0	(24)	(62)	(193)
Reclassification adjustment, securities, tax (expense) benefit	9	2	27	(2)
Other comprehensive income (loss), tax (expense) benefit	9	(22)	(35)	(195)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(215)	(389)	(215)	(389)
Accumulated other comprehensive income (loss), beginning	436	768	362	418
Other comprehensive income (loss) before reclassifications, after-tax	(5)	70	101	413
Reclassification adjustment, after-tax	(17)	(4)	(49)	3
Shareholders' other comprehensive income (loss), net of tax	(22)	66	52	416
Accumulated other comprehensive income (loss), ending	414	834	414	834
Net unrealized appreciation on securities [ Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Realized investment (gains) losses	(26)	(6)	(76)	5
Net unrealized appreciation on derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning		5	4	10
Other comprehensive income (loss) before reclassifications, pre-tax		2	(1)	(3)
Reclassification adjustment, pre-tax		(4)	(3)	(4)
Other comprehensive income (loss), pre-tax		(2)	(4)	(7)
Accumulated other comprehensive income (loss), before tax, ending	0	3	0	3
Accumulated other comprehensive income/loss tax (expense) benefit, beginning		(2)	(1)	(3)
Other comprehensive income (loss), derivatives, before reclassifications, tax (expense) benefit		0	0	1
Reclassification adjustment, derivatives, tax (expense) benefit		1	1	1
Other comprehensive income (loss), tax (expense) benefit		1	1	2
Accumulated other comprehensive income/loss tax (expense) benefit, ending	0	(1)	0	(1)
Accumulated other comprehensive income (loss), beginning		3	3	7
Other comprehensive income (loss) before reclassifications, after-tax		2	(1)	(2)
Reclassification adjustment, after-tax		(3)	(2)	(3)
Shareholders' other comprehensive income (loss), net of tax		(1)	(3)	(5)
Accumulated other comprehensive income (loss), ending	0	2	0	2
Net unrealized appreciation on derivatives [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Realized investment (gains) losses			(3)
Other operating expenses		(4)		(4)
Net translation of foreign currencies [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(249)	(251)	(390)	(295)
Other comprehensive income (loss), pre-tax	38	55	179	99
Accumulated other comprehensive income (loss), before tax, ending	(211)	(196)	(211)	(196)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	19	18	21	21
Other comprehensive income (loss), tax (expense) benefit	(4)	0	(6)	(3)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	15	18	15	18
Accumulated other comprehensive income (loss), beginning	(230)	(233)	(369)	(274)
Shareholders' other comprehensive income (loss), net of tax	34	55	173	96
Accumulated other comprehensive income (loss), ending	(196)	(178)	(196)	(178)
Postretirement benefits liability adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(2,080)	(2,127)	(2,120)	(2,155)
Other comprehensive income (loss) before reclassifications, pre-tax		(3)	2	(7)
Reclassification adjustment, pre-tax	16	16	54	48
Other comprehensive income (loss), pre-tax		13	56	41
Accumulated other comprehensive income (loss), before tax, ending	(2,064)	(2,114)	(2,064)	(2,114)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	728	745	742	754
Other comprehensive income (loss), postretirement benefits, before reclassifications, tax (expense) benefit		0	0	2
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(6)	(4)	(20)	(15)
Other comprehensive income (loss), tax (expense) benefit		(4)	(20)	(13)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	722	741	722	741
Accumulated other comprehensive income (loss), beginning	(1,352)	(1,382)	(1,378)	(1,401)
Other comprehensive income (loss) before reclassifications, after-tax		(3)	2	(5)
Reclassification adjustment, after-tax	10	12	34	33
Shareholders' other comprehensive income (loss), net of tax		9	36	28
Accumulated other comprehensive income (loss), ending	(1,342)	(1,373)	(1,342)	(1,373)
Postretirement benefits liability adjustment [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Other operating expenses	16	16	54	48
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	16	16	48	48
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(6)	(4)	(18)	(15)
Reclassification adjustment, after-tax	10	12	30	33
Reclassification adjustment for amortization of net losses from past experience and prior service costs [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Other operating expenses	$ 16	$ 16	48	$ 48
Reclassification adjustment for settlement losses [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification adjustment, pre-tax			6
Reclassification adjustment, postretirement benefits, tax (expense) benefit			(2)
Reclassification adjustment, after-tax			4
Reclassification adjustment for settlement losses [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Other operating expenses			$ 6