Comprehensive Income - Components of Other Comprehensive Income or Loss (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net unrealized holding gains arising during the period, Before Tax	$ 168	$ 424
Reclassification adjustment for net realized gains included in net income, Before Tax	1	63
Change in unrealized appreciation of fixed maturities and equity securities	167	361
Postretirement benefit gain not yet recognized in net income arising during the year, Before Tax	9
Reclassification adjustment for the amortization of net actuarial loss and prior service cost included in net income, Before Tax	(16)	(9)
Net change in postretirement benefit costs not yet recognized in net income, Before Tax	25	9
Foreign currency translation losses, Before Tax	(270)	(28)
Total other comprehensive income (loss), Before Tax	(78)	342
Net unrealized holding gains arising during the period, Income Tax	58	148
Reclassification adjustment for net realized gains included in net income, Income Tax		22
Net unrealized gains recognized in other comprehensive income, Income Tax	58	126
Postretirement benefit gain not yet recognized in net income arising during the year, Income Tax	3
Reclassification adjustment for the amortization of net actuarial loss and prior service cost included in net income, Income Tax	(5)	(3)
Net change in postretirement benefit costs not yet recognized in net income, Income Tax	8	3
Foreign currency translation losses, Income Tax	(94)	(9)
Total other comprehensive income (loss), Income Tax	(28)	120
Net unrealized holding gains arising during the period, Net of Tax	110	276
Reclassification adjustment for net realized gains included in net income, Net of Tax	1	41
Net unrealized gains recognized in other comprehensive income, Net of Tax	109	235
Postretirement benefit gain not yet recognized in net income arising during the year, Net of Tax	6
Reclassification adjustment for the amortization of net actuarial loss and prior service cost included in net income, Net of Tax	(11)	(6)
Net change in postretirement benefit costs not yet recognized in net income, Net of Tax	17	6
Foreign currency translation losses, Net of Tax	(176)	(19)
Total Other Comprehensive Income (Loss) Net of Tax	$ (50)	$ 222