Remaining Contractual Maturity of Agreements (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	$ 330	$ 553
Securities lending	366	299
Total repurchase agreements and securities lending	696	852
U.S. government, agencies and government-sponsored enterprises
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	330	553
Equity Securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	5	2
Fixed maturity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	361	297
Fixed maturity securities | U.S. government, agencies and government-sponsored enterprises
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	28	36
Fixed maturity securities | Non-U.S. government
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	47	32
Fixed maturity securities | U.S. corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	90	66
Fixed maturity securities | Non-U.S. corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	196	163
Overnight and continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	366	299
Total repurchase agreements and securities lending	366	299
Overnight and continuous | U.S. government, agencies and government-sponsored enterprises
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	0	0
Overnight and continuous | Equity Securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	5	2
Overnight and continuous | Fixed maturity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	361	297
Overnight and continuous | Fixed maturity securities | U.S. government, agencies and government-sponsored enterprises
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	28	36
Overnight and continuous | Fixed maturity securities | Non-U.S. government
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	47	32
Overnight and continuous | Fixed maturity securities | U.S. corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	90	66
Overnight and continuous | Fixed maturity securities | Non-U.S. corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	196	163
Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Total repurchase agreements and securities lending	103	129
Up to 30 days | U.S. government, agencies and government-sponsored enterprises
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	103	129
Up to 30 days | Equity Securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Up to 30 days | Fixed maturity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Up to 30 days | Fixed maturity securities | U.S. government, agencies and government-sponsored enterprises
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Up to 30 days | Fixed maturity securities | Non-U.S. government
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Up to 30 days | Fixed maturity securities | U.S. corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Up to 30 days | Fixed maturity securities | Non-U.S. corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
31 - 90 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Total repurchase agreements and securities lending	70	123
31 - 90 days | U.S. government, agencies and government-sponsored enterprises
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	70	123
31 - 90 days | Equity Securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
31 - 90 days | Fixed maturity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
31 - 90 days | Fixed maturity securities | U.S. government, agencies and government-sponsored enterprises
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
31 - 90 days | Fixed maturity securities | Non-U.S. government
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
31 - 90 days | Fixed maturity securities | U.S. corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
31 - 90 days | Fixed maturity securities | Non-U.S. corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Greater than 90 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Total repurchase agreements and securities lending	157	301
Greater than 90 days | U.S. government, agencies and government-sponsored enterprises
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	157	301
Greater than 90 days | Equity Securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Greater than 90 days | Fixed maturity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Greater than 90 days | Fixed maturity securities | U.S. government, agencies and government-sponsored enterprises
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Greater than 90 days | Fixed maturity securities | Non-U.S. government
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Greater than 90 days | Fixed maturity securities | U.S. corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	0	0
Greater than 90 days | Fixed maturity securities | Non-U.S. corporate
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Securities lending	$ 0	$ 0