Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), beginning	$ (936)
Shareholders' other comprehensive income (loss)	(314)	$ (416)	$ 151
Accumulated other comprehensive income (loss), ending	(1,250)	(936)
Realized investment (gains) losses	(57)	(154)	(213)
Other operating expenses	8,982	8,174	7,595
Net unrealized appreciation on securities [ Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	955	733	1,352
Other comprehensive income (loss) before reclassifications, pre-tax	(389)	249	(498)
Reclassification adjustment, pre-tax	46	(27)	(121)
Other comprehensive income (loss), pre-tax	(343)	222	(619)
Accumulated other comprehensive income (loss), before tax, ending	612	955	733
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	(335)	(256)	(465)
Other comprehensive income (loss), securities, before reclassifications, tax (expense) benefit	157	(89)	166
Reclassification adjustment, securities, tax (expense) benefit	(16)	10	43
Other comprehensive income (loss), tax (expense) benefit	141	(79)	209
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(194)	(335)	(256)
Accumulated other comprehensive income (loss), beginning	620	477	887
Other comprehensive income (loss) before reclassifications, after-tax	(232)	160	(332)
Reclassification adjustment, after-tax	30	(17)	(78)
Shareholders' other comprehensive income (loss)	(202)	143	(410)
Accumulated other comprehensive income (loss), ending	418	620	477
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	(46)	(27)	(121)
Net unrealized appreciation on derivatives [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(12)	(29)	(43)
Other comprehensive income (loss) before reclassifications, pre-tax	10
Reclassification adjustment, pre-tax	12
Other comprehensive income (loss), pre-tax	22	17	14
Accumulated other comprehensive income (loss), before tax, ending	10	(12)	(29)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	4	10	15
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	(7)	(6)	(5)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	(3)	4	10
Accumulated other comprehensive income (loss), beginning	(8)	(19)	(28)
Other comprehensive income (loss) before reclassifications, after-tax	7
Reclassification adjustment, after-tax	8
Shareholders' other comprehensive income (loss)	15	11	9
Accumulated other comprehensive income (loss), ending	7	(8)	(19)
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 translation of foreign currencies [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(71)	91	91
Other comprehensive income (loss), pre-tax	(224)	(162)	0
Accumulated other comprehensive income (loss), before tax, ending	(295)	(71)	91
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	9	(9)	(22)
Other comprehensive income (loss), tax (expense) benefit	12	18	13
Accumulated other comprehensive income/loss tax (expense) benefit, ending	21	9	(9)
Accumulated other comprehensive income (loss), beginning	(62)	82	69
Shareholders' other comprehensive income (loss)	(212)	(144)	13
Accumulated other comprehensive income (loss), ending	(274)	(62)	82
Postretirement benefits liability adjustment [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated other comprehensive income (loss), before tax, beginning	(2,286)	(1,630)	(2,460)
Other comprehensive income (loss) before reclassifications, pre-tax	63	(716)	779
Reclassification adjustment, pre-tax	68	60	51
Other comprehensive income (loss), pre-tax	131	(656)	830
Accumulated other comprehensive income (loss), before tax, ending	(2,155)	(2,286)	(1,630)
Accumulated other comprehensive income/loss tax (expense) benefit, beginning	800	570	861
Other comprehensive income (loss), postretirement benefits, before reclassifications, tax (expense) benefit	(23)	250	(273)
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(23)	(20)	(18)
Other comprehensive income (loss), tax (expense) benefit	(46)	230	(291)
Accumulated other comprehensive income/loss tax (expense) benefit, ending	754	800	570
Accumulated other comprehensive income (loss), beginning	(1,486)	(1,060)	(1,599)
Other comprehensive income (loss) before reclassifications, after-tax	40	(466)	506
Reclassification adjustment, after-tax	45	40	33
Shareholders' other comprehensive income (loss)	85	(426)	539
Accumulated other comprehensive income (loss), ending	(1,401)	(1,486)	(1,060)
Postretirement benefits liability adjustment [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Other operating expenses	68	60	51
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	68	54	70
Reclassification adjustment, postretirement benefits, tax (expense) benefit	(23)	(18)	(25)
Reclassification adjustment, after-tax	45	36	45
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	$ 68	54	70
Reclassification adjustment for curtailment gain [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Reclassification adjustment, pre-tax			(19)
Reclassification adjustment, postretirement benefits, tax (expense) benefit			7
Reclassification adjustment, after-tax			(12)
Reclassification adjustment for curtailment gain [Member] | Reclassification Out Of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Other operating expenses			$ (19)
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