Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	$ (1,382)
Shareholders' other comprehensive income (loss), net of tax	130	$ 262
Accumulated other comprehensive income (loss), ending	(1,252)
Realized investment (gains) losses	(46)	32
Other operating expenses	2,530	2,321
Net unrealized appreciation on securities [ Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	542	612
Other comprehensive income (loss) before reclassifications, pre-tax	53	227
Reclassification adjustment, pre-tax	(35)	28
Other comprehensive income (loss), pre-tax	18	255
Accumulated other comprehensive income (loss), before tax, ending	560	867
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(180)	(194)
Other comprehensive income (loss), securities, before reclassifications, tax (expense) benefit	(24)	(72)
Reclassification adjustment, securities, tax (expense) benefit	13	(10)
Other comprehensive income (loss), tax (expense) benefit	(11)	(82)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(191)	(276)
Accumulated other comprehensive income (loss), beginning	362	418
Other comprehensive income (loss) before reclassifications, after-tax	29	155
Reclassification adjustment, after-tax	(22)	18
Shareholders' other comprehensive income (loss), net of tax	7	173
Accumulated other comprehensive income (loss), ending	369	591
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	(35)	28
Net unrealized appreciation on derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	4	10
Other comprehensive income (loss) before reclassifications, pre-tax		(4)
Reclassification adjustment, pre-tax	(4)
Accumulated other comprehensive income (loss), before tax, ending	0	6
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(1)	(3)
Other comprehensive income (loss), derivatives, before reclassifications, tax (expense) benefit		1
Reclassification adjustment, derivatives, tax (expense) benefit	1
Accumulated other comprehensive income/loss tax (expense) benefit, ending	0	(2)
Accumulated other comprehensive income (loss), beginning	3	7
Other comprehensive income (loss) before reclassifications, after-tax		(3)
Reclassification adjustment, after-tax	(3)
Accumulated other comprehensive income (loss), ending	0	4
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	(390)	(295)
Other comprehensive income (loss), pre-tax	111	83
Accumulated other comprehensive income (loss), before tax, ending	(279)	(212)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	21	21
Other comprehensive income (loss), tax (expense) benefit	1	(2)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	22	19
Accumulated other comprehensive income (loss), beginning	(369)	(274)
Shareholders' other comprehensive income (loss), net of tax	112	81
Accumulated other comprehensive income (loss), ending	(257)	(193)
Postretirement benefits liability adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(2,120)	(2,155)
Reclassification adjustment, pre-tax	21	16
Other comprehensive income (loss), pre-tax	21
Accumulated other comprehensive income (loss), before tax, ending	(2,099)	(2,139)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	742	754
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(7)	(5)
Other comprehensive income (loss), tax (expense) benefit	(7)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	735	749
Accumulated other comprehensive income (loss), beginning	(1,378)	(1,401)
Reclassification adjustment, after-tax	14	11
Shareholders' other comprehensive income (loss), net of tax	14
Accumulated other comprehensive income (loss), ending	(1,364)	(1,390)
Postretirement benefits liability adjustment [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Other operating expenses	21	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	15	16
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(5)	(5)
Reclassification adjustment, after-tax	10	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	15	$ 16
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