Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Net unrealized appreciation (depreciation), securities, pre-tax:
Net unrealized appreciation (depreciation) on securities arising during the period pre-tax	$ (384)	$ 98	$ (420)	$ 149
Reclassification adjustment for (gains) losses included in shareholders' net income pre-tax	(23)	(3)	(93)	(19)
Net unrealized appreciation (depreciation), securities pre-tax	(407)	95	(513)	130
Net unrealized appreciation (depreciation) derivatives pre-tax	10	6	14	0
Net translation of foreign currencies pre-tax	(23)	(45)	(92)	(2)
Postretirement benefits liability adjustment, pre-tax:
Reclassification adjustment for amortization of net losses from past experience and prior service costs pre-tax	17	12	34	24
Reclassification adjustment for curtailment or settlement (other operating expenses)			(19)	6
Total reclassification adjustments to shareholders' net income (other operating expenses)			15	30
Net change due to valuation update and plan amedments	16	24	79	24
Net postretirement benefits liability adjustment pre-tax	33	36	94	54
Net unrealized appreciation, securities, tax (expense) benefit:
Net unrealized appreciation (depreciation) on securities arising during the period tax (expense) benefit	130	(32)	142	(48)
Reclassification adjustment for (gains) losses included in shareholders' net income tax (expense) benefit	8	1	32	6
Net unrealized appreciation (depreciation), securities tax (expense) benefit	138	(31)	174	(42)
Net unrealized appreciation (depreciation) derivatives tax (expense) benefit	(4)	(1)	(5)	0
Net translation of foreign currencies tax (expense) benefit	7	2	18	(6)
Postretirement benefits liability adjustment, tax (expense) benefit:
Reclassification adjustment for amortization of net losses from past experience and prior service costs tax (expense) benefit	(6)	(3)	(12)	(8)
Reclassification adjustment for curtailment or settlement (other operating expenses)			7	(2)
Total reclassification adjustments to shareholders' net income (other operating expenses)			(5)	(10)
Net change due to valuation update and plan amedments	(6)	(8)	(28)	(8)
Net postretirement benefits liability adjustment tax (expense) benefit	(12)	(11)	(33)	(18)
Net realized appreciation (depreciation), securities after-tax:
Net unrealized appreciation (depreciation) on securities arising during the period after-tax	(254)	66	(278)	101
Reclassification adjustment for (gains) losses included in shareholders' net income after-tax	(15)	(2)	(61)	(13)
Net unrealized appreciation (depreciation), securities after-tax	(269)	64	(339)	88
Net unrealized appreciation (depreciation), derivatives after-tax	6	5	9	0
Net translation of foreign currencies after-tax	(16)	(43)	(74)	(8)
Postretirement benefits liability adjustment, after-tax:
Reclassification adjustment for amortization of net losses from past experience and prior service costs after-tax	11	9	22	16
Reclassification adjustment for curtailment or settlement (other operating expenses)			(12)	4
Total reclassification adjustments to shareholders' net income (other operating expenses)			10	20
Net change due to valuation update and plan amedments	10	16	51	16
Net postretirement benefits liability adjustment after-tax	$ 21	$ 25	$ 61	$ 36