Comprehensive Income - Components of Other Comprehensive Income or Loss (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unrealized holding gains arising during the year, Before tax	$ 556	$ 1,029	$ 230
Unrealized other-than-temporary impairment losses arising during the year, Before tax	(2)	(1)	(3)
Reclassification adjustment for realized gains included in net income, Before tax	127	81	110
Net unrealized gains recognized in other comprehensive income or loss, Before tax	427	947	117
Foreign currency translation gains (losses), Before tax	(16)	6	(28)
Change in postretirement benefit costs not yet recognized in net income, Before tax	(45)	(329)	20
Total other comprehensive income, Before tax	366	624	109
Unrealized holding gains arising during the year, Income tax	195	359	81
Unrealized other-than-temporary impairment losses arising during the year, Income tax	(1)		(1)
Reclassification adjustment for realized gains included in net income, Income tax	44	28	39
Net unrealized gains recognized in other comprehensive income or loss, Income tax	150	331	41
Foreign currency translation gains (losses), Income tax	(5)	2	(10)
Change in postretirement benefit costs not yet recognized in net income, Income tax	(15)	(114)	8
Total other comprehensive income, Income tax	130	219	39
Unrealized holding gains arising during the year, Net of tax	361	670	149
Unrealized other-than-temporary impairment losses arising during the year, Net of tax	(1)	(1)	(2)
Reclassification adjustment for realized gains included in net income, Net of tax	83	53	71
Net unrealized gains recognized in other comprehensive income or loss, Net of tax	277	616	76
Foreign currency translation gains (losses), Net of tax	(11)	4	(18)
Change in postretirement benefit costs not yet recognized in net income, Net of tax	(30)	(215)	12
Total Other Comprehensive Income, Net of Tax	$ 236	$ 405	$ 70