Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net unrealized appreciation, securities pre-tax:
Net unrealized appreciation (depreciation) on securities arising during the period pre-tax	$ 271	$ 366	$ 319
Reclassification adjustment for (gains) losses included in shareholders' net income pre-tax	(52)	(49)	(92)
Net unrealized appreciation (depreciation), securities pre-tax	219	317	227
Net unrealized appreciation (depreciation) derivatives pre-tax	(7)	1	8
Net translation of foreign currencies pre-tax	78	(21)	40
Postretirement benefits liability adjustment pre-tax:
Reclassification adjustment for amortization of net losses from past experience and prior service costs pre-tax	52	22	10
Net change arising from assumption and plan changes and experience	(181)	(580)	(311)
Net postretirement benefits liability adjustment pre-tax	(129)	(558)	(301)
Net unrealized appreciation, securities tax (expense) benefit:
Net unrealized appreciation (depreciation) on securities arising during the period tax (expense) benefit	(90)	(127)	(109)
Reclasification adjustment for (gains) losses included in shareholders' net income tax (expense) benefit	18	18	32
Net unrealized appreciation (depreciation), securities tax (expense) benefit	(72)	(109)	(77)
Net unrealized appreciation (depreciation) derivatives tax (expense) benefit	2	0	(2)
Net translation of foreign currencies tax (expense) benefit	(12)	(1)	(7)
Postretirement benefits liability adjustment tax (expense) benefit:
Reclassification adjustment for amortization of net losses from past experience and prior service costs tax (expense) benefit	(18)	(7)	(4)
Net change arising from assumption and plan changes and experience tax (expense) benefit	55	205	116
Net postretirement benefits liability adjustment tax (expense) benefit	37	198	112
Net realized appreciation, securities after-tax:
Net unrealized appreciation (depreciation) on securities arising during the period after-tax	181	239	210
Reclassification adjustment for (gains) losses included in shareholders' net income after-tax	(34)	(31)	(60)
Net unrealized appreciation (depreciation), securities after-tax	147	208	150
Net unrealized appreciation (depreciation), derivatives after-tax	(5)	1	6
Net translation of foreign currencies after-tax	66	(22)	33
Postretirement benefits liability adjustment after-tax:
Reclassification adjustment for amortization of net losses from past experience and prior service costs after-tax	34	15	6
Net change arising from assumption and plan changes and experience after-tax	(126)	(375)	(195)
Net postretirement benefits liability adjustment after-tax	$ (92)	$ (360)	$ (189)