Comprehensive Income - Components of Other Comprehensive Income or Loss (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net unrealized holding gains (losses) arising during the year, Before Tax	$ 1,032	$ (1,000)	$ 556
Unrealized other-than-temporary impairment losses arising during the year, Before Tax			(2)
Reclassification adjustment for net realized gains included in net income, Before Tax	220	212	127
Net unrealized gains (losses) recognized in other comprehensive income or loss, Before Tax	812	(1,212)	427
Postretirement benefit gain (loss) not yet recognized in net income arising during the year, Before Tax	(550)	622	(128)
Reclassification adjustment for the amortization of net actuarial loss and prior service cost included in net income, Before Tax	(35)	(93)	(83)
Net change in postretirement benefit costs not yet recognized in net income, Before Tax	(515)	715	(45)
Foreign currency translation losses, Before Tax	(180)	(111)	(16)
Total other comprehensive income (loss), Before Tax	117	(608)	366
Net unrealized holding gains (losses) arising during the year, Income Tax	361	(350)	195
Unrealized other-than-temporary impairment losses arising during the year, Income Tax			(1)
Reclassification adjustment for net realized gains included in net income, Income Tax	77	74	44
Net unrealized gains (losses) recognized in other comprehensive income or loss, Income Tax	284	(424)	150
Postretirement benefit gain (loss) not yet recognized in net income arising during the year, Income Tax	(192)	218	(45)
Reclassification adjustment for the amortization of net actuarial loss and prior service cost included in net income, Income Tax	(13)	(33)	(30)
Net change in postretirement benefit costs not yet recognized in net income, Income Tax	(179)	251	(15)
Foreign currency translation losses, Income Tax	(63)	(39)	(5)
Total other comprehensive income (loss), Income Tax	42	(212)	130
Net unrealized holding gains (losses) arising during the year, Net of Tax	671	(650)	361
Unrealized other-than-temporary impairment losses arising during the year, Net of Tax			(1)
Reclassification adjustment for net realized gains included in net income, Net of Tax	143	138	83
Net unrealized gains (losses) recognized in other comprehensive income or loss, Net of Tax	528	(788)	277
Postretirement benefit gain (loss) not yet recognized in net income arising during the period, Net of Tax	(358)	404	(83)
Reclassification adjustment for the amortization of net actuarial loss and prior service cost included in net income, Net of Tax	(22)	(60)	(53)
Net change in postretirement benefit costs not yet recognized in net income, Net of Tax	(336)	464	(30)
Foreign currency translation losses, Net of Tax	(117)	(72)	(11)
Total Other Comprehensive Income (Loss) Net of Tax	$ 75	$ (396)	$ 236