Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	$ (1,250)
Shareholders' other comprehensive income (loss), net of tax	(132)	$ (314)	$ (416)
Accumulated other comprehensive income (loss), ending	(1,382)	(1,250)
Realized investment (gains) losses	(169)	(57)	(154)
Other operating expenses	9,584	8,982	8,174
Net unrealized appreciation on securities [ Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	612	955	733
Other comprehensive income (loss) before reclassifications, pre-tax	(48)	(389)	249
Reclassification adjustment, pre-tax	(22)	46	(27)
Other comprehensive income (loss), pre-tax	(70)	(343)	222
Accumulated other comprehensive income (loss), before tax, ending	542	612	955
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(194)	(335)	(256)
Other comprehensive income (loss), securities, before reclassifications, tax (expense) benefit	6	157	(89)
Reclassification adjustment, securities, tax (expense) benefit	8	(16)	10
Other comprehensive income (loss), tax (expense) benefit	14	141	(79)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(180)	(194)	(335)
Accumulated other comprehensive income (loss), beginning	418	620	477
Other comprehensive income (loss) before reclassifications, after-tax	(42)	(232)	160
Reclassification adjustment, after-tax	(14)	30	(17)
Shareholders' other comprehensive income (loss), net of tax	(56)	(202)	143
Accumulated other comprehensive income (loss), ending	362	418	620
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	(22)	46	(27)
Net unrealized appreciation on derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	10	(12)	(29)
Other comprehensive income (loss) before reclassifications, pre-tax		10
Reclassification adjustment, pre-tax		12
Other comprehensive income (loss), pre-tax	(6)	22	17
Accumulated other comprehensive income (loss), before tax, ending	4	10	(12)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(3)	4	10
Other comprehensive income (loss), derivatives, before reclassifications, tax (expense) benefit		(3)
Reclassification adjustment, derivatives, tax (expense) benefit		(4)
Other comprehensive income (loss), tax (expense) benefit	2	(7)	(6)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(1)	(3)	4
Accumulated other comprehensive income (loss), beginning	7	(8)	(19)
Other comprehensive income (loss) before reclassifications, after-tax		7
Reclassification adjustment, after-tax		8
Shareholders' other comprehensive income (loss), net of tax	(4)	15	11
Accumulated other comprehensive income (loss), ending	3	7	(8)
Net unrealized appreciation on derivatives [Member] | Other Operating Expenses [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification adjustment, pre-tax	1
Reclassification adjustment, derivatives, tax (expense) benefit	0
Reclassification adjustment, after-tax	1
Net unrealized appreciation on derivatives [Member] | Net Realized Investment Gains [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification adjustment, pre-tax	(7)
Reclassification adjustment, derivatives, tax (expense) benefit	2
Reclassification adjustment, after-tax	(5)
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
Net unrealized appreciation on derivatives [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member] | Other Operating Expenses [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Other operating expenses	1
Net unrealized appreciation on derivatives [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member] | Net Realized Investment Gains [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Realized investment (gains) losses	(7)
Net translation of foreign currencies [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(295)	(71)	91
Other comprehensive income (loss), pre-tax	(95)	(224)	(162)
Accumulated other comprehensive income (loss), before tax, ending	(390)	(295)	(71)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	21	9	(9)
Other comprehensive income (loss), tax (expense) benefit	0	12	18
Accumulated other comprehensive income/loss tax (expense) benefit, ending	21	21	9
Accumulated other comprehensive income (loss), beginning	(274)	(62)	82
Shareholders' other comprehensive income (loss), net of tax	(95)	(212)	(144)
Accumulated other comprehensive income (loss), ending	(369)	(274)	(62)
Postretirement benefits liability adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(2,155)	(2,286)	(1,630)
Other comprehensive income (loss) before reclassifications, pre-tax	(29)	63	(716)
Reclassification adjustment, pre-tax	64	68	60
Other comprehensive income (loss), pre-tax	35	131	(656)
Accumulated other comprehensive income (loss), before tax, ending	(2,120)	(2,155)	(2,286)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	754	800	570
Other comprehensive income (loss), postretirement benefits, before reclassifications, tax (expense) benefit	10	(23)	250
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(22)	(23)	(20)
Other comprehensive income (loss), tax (expense) benefit	(12)	(46)	230
Accumulated other comprehensive income/loss tax (expense) benefit, ending	742	754	800
Accumulated other comprehensive income (loss), beginning	(1,401)	(1,486)	(1,060)
Other comprehensive income (loss) before reclassifications, after-tax	(19)	40	(466)
Reclassification adjustment, after-tax	42	45	40
Shareholders' other comprehensive income (loss), net of tax	23	85	(426)
Accumulated other comprehensive income (loss), ending	(1,378)	(1,401)	(1,486)
Postretirement benefits liability adjustment [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Other operating expenses	64	68	60
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	64	68	54
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(22)	(23)	(18)
Reclassification adjustment, after-tax	42	45	36
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	$ 64	$ 68	54
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