Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	$ (1,250)
Shareholders' other comprehensive income (loss)	262	$ 2
Accumulated other comprehensive income (loss), ending	(988)
Realized investment (gains) losses	32	(73)
Other operating expenses	2,321	2,204
Net unrealized appreciation on securities [ Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	612	955
Other comprehensive income (loss) before reclassifications, pre-tax	227	148
Reclassification adjustment, pre-tax	28	(20)
Other comprehensive income (loss), pre-tax	255	128
Accumulated other comprehensive income (loss), before tax, ending	867	1,083
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(194)	(335)
Other comprehensive income (loss), securities, before reclassifications, tax (expense) benefit	(72)	(47)
Reclassification adjustment, securities, tax (expense) benefit	(10)	7
Other comprehensive income (loss), tax (expense) benefit	(82)	(40)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(276)	(375)
Accumulated other comprehensive income (loss), beginning	418	620
Other comprehensive income (loss) before reclassifications, after-tax	155	101
Reclassification adjustment, after-tax	18	(13)
Shareholders' other comprehensive income (loss)	173	88
Accumulated other comprehensive income (loss), ending	591	708
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	28	(20)
Net unrealized appreciation on derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	10	(12)
Other comprehensive income (loss), pre-tax	(4)	10
Accumulated other comprehensive income (loss), before tax, ending	6	(2)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(3)	4
Other comprehensive income (loss), tax (expense) benefit	1	(3)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(2)	1
Accumulated other comprehensive income (loss), beginning	7	(8)
Shareholders' other comprehensive income (loss)	(3)	7
Accumulated other comprehensive income (loss), ending	4	(1)
Net translation of foreign currencies [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(295)	(71)
Other comprehensive income (loss), pre-tax	83	(109)
Accumulated other comprehensive income (loss), before tax, ending	(212)	(180)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	21	9
Other comprehensive income (loss), tax (expense) benefit	(2)	5
Accumulated other comprehensive income/loss tax (expense) benefit, ending	19	14
Accumulated other comprehensive income (loss), beginning	(274)	(62)
Shareholders' other comprehensive income (loss)	81	(104)
Accumulated other comprehensive income (loss), ending	(193)	(166)
Postretirement benefits liability adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(2,155)	(2,286)
Reclassification adjustment, pre-tax	16	16
Accumulated other comprehensive income (loss), before tax, ending	(2,139)	(2,270)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	754	800
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(5)	(5)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	749	795
Accumulated other comprehensive income (loss), beginning	(1,401)	(1,486)
Reclassification adjustment, after-tax	11	11
Accumulated other comprehensive income (loss), ending	(1,390)	(1,475)
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
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
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(5)	(5)
Reclassification adjustment, after-tax	11	11
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