Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning			$ (1,250)
Shareholders' other comprehensive income (loss)	$ 144	$ (191)	406	$ (189)
Accumulated other comprehensive income (loss), ending	(844)		(844)
Realized investment (gains) losses	(67)	(21)	(35)	(94)
Other operating expenses	2,289	2,212	4,610	4,416
Net unrealized appreciation on securities [ Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	867	1,083	612	955
Other comprehensive income (loss) before reclassifications, pre-tax	285	(337)	512	(189)
Reclassification adjustment, pre-tax	(17)	(13)	11	(33)
Other comprehensive income (loss), pre-tax	268	(350)	523	(222)
Accumulated other comprehensive income (loss), before tax, ending	1,135	733	1,135	733
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(276)	(375)	(194)	(335)
Other comprehensive income (loss), securities, before reclassifications, tax (expense) benefit	(97)	112	(169)	64
Reclassification adjustment, securities, tax (expense) benefit	6	4	(4)	12
Other comprehensive income (loss), tax (expense) benefit	(91)	116	(173)	76
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(367)	(259)	(367)	(259)
Accumulated other comprehensive income (loss), beginning	591	708	418	620
Other comprehensive income (loss) before reclassifications, after-tax	188	(225)	343	(125)
Reclassification adjustment, after-tax	(11)	(9)	7	(21)
Shareholders' other comprehensive income (loss)	177	(234)	350	(146)
Accumulated other comprehensive income (loss), ending	768	474	768	474
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	(17)	(13)	11	(33)
Net unrealized appreciation on derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	6	(2)	10	(12)
Other comprehensive income (loss) before reclassifications, pre-tax	(1)	(3)	(5)	7
Reclassification adjustment, pre-tax		12		12
Other comprehensive income (loss), pre-tax		9		19
Accumulated other comprehensive income (loss), before tax, ending	5	7	5	7
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(2)	1	(3)	4
Other comprehensive income (loss), derivatives, before reclassifications, tax (expense) benefit	0	0	1	(3)
Reclassification adjustment, derivatives, tax (expense) benefit		(4)		(4)
Other comprehensive income (loss), tax (expense) benefit		(4)		(7)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(2)	(3)	(2)	(3)
Accumulated other comprehensive income (loss), beginning	4	(1)	7	(8)
Other comprehensive income (loss) before reclassifications, after-tax	(1)	(3)	(4)	4
Reclassification adjustment, after-tax		8		8
Shareholders' other comprehensive income (loss)		5		12
Accumulated other comprehensive income (loss), ending	3	4	3	4
Net unrealized appreciation on derivatives [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Other operating expenses		12		12
Net translation of foreign currencies [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(212)	(180)	(295)	(71)
Other comprehensive income (loss), pre-tax	(39)	15	44	(94)
Accumulated other comprehensive income (loss), before tax, ending	(251)	(165)	(251)	(165)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	19	14	21	9
Other comprehensive income (loss), tax (expense) benefit	(1)	3	(3)	8
Accumulated other comprehensive income/loss tax (expense) benefit, ending	18	17	18	17
Accumulated other comprehensive income (loss), beginning	(193)	(166)	(274)	(62)
Shareholders' other comprehensive income (loss)	(40)	18	41	(86)
Accumulated other comprehensive income (loss), ending	(233)	(148)	(233)	(148)
Postretirement benefits liability adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(2,139)	(2,270)	(2,155)	(2,286)
Other comprehensive income (loss) before reclassifications, pre-tax	(4)	12	(4)	12
Reclassification adjustment, pre-tax	16	18	32	34
Other comprehensive income (loss), pre-tax	12	30	28	46
Accumulated other comprehensive income (loss), before tax, ending	(2,127)	(2,240)	(2,127)	(2,240)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	749	795	754	800
Other comprehensive income (loss), postretirement benefits, before reclassifications, tax (expense) benefit	2	(3)	2	(3)
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(6)	(7)	(11)	(12)
Other comprehensive income (loss), tax (expense) benefit	(4)	(10)	(9)	(15)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	745	785	745	785
Accumulated other comprehensive income (loss), beginning	(1,390)	(1,475)	(1,401)	(1,486)
Other comprehensive income (loss) before reclassifications, after-tax	(2)	9	(2)	9
Reclassification adjustment, after-tax	10	11	21	22
Shareholders' other comprehensive income (loss)	8	20	19	31
Accumulated other comprehensive income (loss), ending	(1,382)	(1,455)	(1,382)	(1,455)
Postretirement benefits liability adjustment [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Other operating expenses	16	18	32	34
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	18	32	34
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(6)	(7)	(11)	(12)
Reclassification adjustment, after-tax	10	11	21	22
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	$ 18	$ 32	$ 34