Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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	$ 106	$ 144	236	$ 406
Accumulated other comprehensive income (loss), ending	(1,146)		(1,146)
Realized investment (gains) losses	(51)	(67)	(97)	(35)
Other operating expenses	2,271	2,289	4,801	4,610
Net unrealized appreciation on securities [ Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	560	867	542	612
Other comprehensive income (loss) before reclassifications, pre-tax	115	285	168	512
Reclassification adjustment, pre-tax	(15)	(17)	(50)	11
Other comprehensive income (loss), pre-tax	100	268	118	523
Accumulated other comprehensive income (loss), before tax, ending	660	1,135	660	1,135
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(191)	(276)	(180)	(194)
Other comprehensive income (loss), securities, before reclassifications, tax (expense) benefit	(38)	(97)	(62)	(169)
Reclassification adjustment, securities, tax (expense) benefit	5	6	18	(4)
Other comprehensive income (loss), tax (expense) benefit	(33)	(91)	(44)	(173)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(224)	(367)	(224)	(367)
Accumulated other comprehensive income (loss), beginning	369	591	362	418
Other comprehensive income (loss) before reclassifications, after-tax	77	188	106	343
Reclassification adjustment, after-tax	(10)	(11)	(32)	7
Shareholders' other comprehensive income (loss), net of tax	67	177	74	350
Accumulated other comprehensive income (loss), ending	436	768	436	768
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	(15)	(17)	(50)	11
Net unrealized appreciation on derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning		6	4	10
Other comprehensive income (loss) before reclassifications, pre-tax		(1)		(5)
Reclassification adjustment, pre-tax			(4)
Accumulated other comprehensive income (loss), before tax, ending	0	5	0	5
Accumulated other comprehensive income/loss tax (expense) benefit, beginning		(2)	(1)	(3)
Other comprehensive income (loss), derivatives, before reclassifications, tax (expense) benefit		0		1
Reclassification adjustment, derivatives, tax (expense) benefit			1
Accumulated other comprehensive income/loss tax (expense) benefit, ending	0	(2)	0	(2)
Accumulated other comprehensive income (loss), beginning		4	3	7
Other comprehensive income (loss) before reclassifications, after-tax		(1)		(4)
Reclassification adjustment, after-tax			(3)
Accumulated other comprehensive income (loss), ending	0	3	0	3
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			(4)
Net translation of foreign currencies [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(279)	(212)	(390)	(295)
Other comprehensive income (loss), pre-tax	30	(39)	141	44
Accumulated other comprehensive income (loss), before tax, ending	(249)	(251)	(249)	(251)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	22	19	21	21
Other comprehensive income (loss), tax (expense) benefit	(3)	(1)	(2)	(3)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	19	18	19	18
Accumulated other comprehensive income (loss), beginning	(257)	(193)	(369)	(274)
Shareholders' other comprehensive income (loss), net of tax	27	(40)	139	41
Accumulated other comprehensive income (loss), ending	(230)	(233)	(230)	(233)
Postretirement benefits liability adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(2,099)	(2,139)	(2,120)	(2,155)
Other comprehensive income (loss) before reclassifications, pre-tax	2	(4)	2	(4)
Reclassification adjustment, pre-tax	17	16	38	32
Other comprehensive income (loss), pre-tax	19	12	40	28
Accumulated other comprehensive income (loss), before tax, ending	(2,080)	(2,127)	(2,080)	(2,127)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	735	749	742	754
Other comprehensive income (loss), postretirement benefits, before reclassifications, tax (expense) benefit	0	2	0	2
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(7)	(6)	(14)	(11)
Other comprehensive income (loss), tax (expense) benefit	(7)	(4)	(14)	(9)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	728	745	728	745
Accumulated other comprehensive income (loss), beginning	(1,364)	(1,390)	(1,378)	(1,401)
Other comprehensive income (loss) before reclassifications, after-tax	2	(2)	2	(2)
Reclassification adjustment, after-tax	10	10	24	21
Shareholders' other comprehensive income (loss), net of tax	12	8	26	19
Accumulated other comprehensive income (loss), ending	(1,352)	(1,382)	(1,352)	(1,382)
Postretirement benefits liability adjustment [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Other operating expenses	17	16	38	32
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	17	16	32	32
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(7)	(6)	(12)	(11)
Reclassification adjustment, after-tax	10	10	20	21
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	$ 17	$ 16	32	$ 32
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