Comprehensive Income - Components of Other Comprehensive Income or Loss (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net unrealized holding losses arising during the period, Before Tax	$ (956)		$ (886)
Reclassification adjustment for net realized gains (Losses) included in net income, Before Tax	120		184
Net unrealized losses recognized in other comprehensive income or loss, Before Tax	(1,076)		(1,070)
Postretirement benefit costs not yet recognized in net income arising during the period, Before Tax	(1)		1
Reclassification adjustment for the amortization of net actuarial loss and prior service cost, Before Tax	(23)		(47)
Net change in postretirement benefit costs not yet recognized in net income, Before Tax	22		48
Foreign currency translation gains (losses), Before Tax	42		(35)
Total other comprehensive loss, Before Tax	(1,012)		(1,057)
Net unrealized holding losses arising during the period, Income Tax	(334)		(310)
Reclassification adjustment for net realized gains included in net income, Income Tax	42		64
Net unrealized losses recognized in other comprehensive income or loss, Income Tax	(376)		(374)
Postretirement benefit costs not yet recognized in net income arising during the period, Income Tax	(1)
Reclassification adjustment for the amortization of net actuarial loss and prior service cost, Income Tax	(9)		(17)
Net change in postretirement benefit costs not yet recognized in net income, Income Tax	8		17
Foreign currency translation gains (losses), Income Tax	14		(13)
Total other comprehensive loss, Income Tax	(354)		(370)
Net unrealized holding losses arising during the period, Net of Tax	(622)		(576)
Reclassification adjustment for net realized gains included in net income, Net of Tax	78		120
Net unrealized losses recognized in other comprehensive income or loss, Net of Tax	(700)	67	(696)	121
Postretirement benefit costs not yet recognized in net income arising during the period, Net of Tax			1
Reclassification adjustment for the amortization of net actuarial loss and prior service cost, Net of Tax	(14)		(30)
Net change in postretirement benefit costs not yet recognized in net income, Net of Tax	14	14	31	28
Foreign currency translation gains (losses), Net of Tax	28	(28)	(22)	(37)
Total Other Comprehensive Income (Loss) Net of Tax	$ (658)	$ 54	$ (687)	$ 114