Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning			$ (1,250)
Shareholders' other comprehensive income (loss)	$ 129	$ (90)	535	$ (279)
Accumulated other comprehensive income (loss), ending	(715)		(715)
Realized investment (gains) losses	(75)	(10)	(110)	(104)
Other operating expenses	2,428	2,171	7,038	6,587
Net unrealized appreciation on securities [ Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	1,135	733	612	955
Other comprehensive income (loss) before reclassifications, pre-tax	94	(37)	606	(226)
Reclassification adjustment, pre-tax	(6)	29	5	(4)
Other comprehensive income (loss), pre-tax	88	(8)	611	(230)
Accumulated other comprehensive income (loss), before tax, ending	1,223	725	1,223	725
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(367)	(259)	(194)	(335)
Other comprehensive income (loss), securities, before reclassifications, tax (expense) benefit	(24)	26	(193)	90
Reclassification adjustment, securities, tax (expense) benefit	2	(10)	(2)	2
Other comprehensive income (loss), tax (expense) benefit	(22)	16	(195)	92
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(389)	(243)	(389)	(243)
Accumulated other comprehensive income (loss), beginning	768	474	418	620
Other comprehensive income (loss) before reclassifications, after-tax	70	(11)	413	(136)
Reclassification adjustment, after-tax	(4)	19	3	(2)
Shareholders' other comprehensive income (loss)	66	8	416	(138)
Accumulated other comprehensive income (loss), ending	834	482	834	482
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	(6)	29	5	(4)
Net unrealized appreciation on derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	5	7	10	(12)
Other comprehensive income (loss) before reclassifications, pre-tax	2	4	(3)	11
Reclassification adjustment, pre-tax	(4)		(4)	12
Other comprehensive income (loss), pre-tax	(2)		(7)	23
Accumulated other comprehensive income (loss), before tax, ending	3	11	3	11
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(2)	(3)	(3)	4
Other comprehensive income (loss), derivatives, before reclassifications, tax (expense) benefit	0	(1)	1	(4)
Reclassification adjustment, derivatives, tax (expense) benefit	1		1	(4)
Other comprehensive income (loss), tax (expense) benefit	1		2	(8)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(1)	(4)	(1)	(4)
Accumulated other comprehensive income (loss), beginning	3	4	7	(8)
Other comprehensive income (loss) before reclassifications, after-tax	2	3	(2)	7
Reclassification adjustment, after-tax	(3)		(3)	8
Shareholders' other comprehensive income (loss)	(1)		(5)	15
Accumulated other comprehensive income (loss), ending	2	7	2	7
Net unrealized appreciation on derivatives [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Other operating expenses	(4)		(4)	12
Net translation of foreign currencies [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(251)	(165)	(295)	(71)
Other comprehensive income (loss), pre-tax	55	(113)	99	(207)
Accumulated other comprehensive income (loss), before tax, ending	(196)	(278)	(196)	(278)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	18	17	21	9
Other comprehensive income (loss), tax (expense) benefit	0	1	(3)	9
Accumulated other comprehensive income/loss tax (expense) benefit, ending	18	18	18	18
Accumulated other comprehensive income (loss), beginning	(233)	(148)	(274)	(62)
Shareholders' other comprehensive income (loss)	55	(112)	96	(198)
Accumulated other comprehensive income (loss), ending	(178)	(260)	(178)	(260)
Postretirement benefits liability adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(2,127)	(2,240)	(2,155)	(2,286)
Other comprehensive income (loss) before reclassifications, pre-tax	(3)		(7)	12
Reclassification adjustment, pre-tax	16	16	48	50
Other comprehensive income (loss), pre-tax	13		41	62
Accumulated other comprehensive income (loss), before tax, ending	(2,114)	(2,224)	(2,114)	(2,224)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	745	785	754	800
Other comprehensive income (loss), postretirement benefits, before reclassifications, tax (expense) benefit	0		2	(3)
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(4)	(5)	(15)	(17)
Other comprehensive income (loss), tax (expense) benefit	(4)		(13)	(20)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	741	780	741	780
Accumulated other comprehensive income (loss), beginning	(1,382)	(1,455)	(1,401)	(1,486)
Other comprehensive income (loss) before reclassifications, after-tax	(3)		(5)	9
Reclassification adjustment, after-tax	12	11	33	33
Shareholders' other comprehensive income (loss)	9		28	42
Accumulated other comprehensive income (loss), ending	(1,373)	(1,444)	(1,373)	(1,444)
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	48	50
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	50
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(4)	(5)	(15)	(17)
Reclassification adjustment, after-tax	12	11	33	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	$ 50