Borrowings - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Feb. 28, 2015	Dec. 31, 2014	Feb. 27, 2014
Schedule Of Borrowings [Line Items]
Interest rate of debt instrument				8.50%
Junior subordinated debentures	$ 513
Agreements to repurchase securities	1,400		$ 1,400
Collateral posted	$ 1,500		$ 1,500
Senior Notes Due 2020 [Member]
Schedule Of Borrowings [Line Items]
Interest rate of debt instrument		2.10%
Fixed rate senior notes issued		$ 750
Senior Notes Due 2025 [Member]
Schedule Of Borrowings [Line Items]
Interest rate of debt instrument		2.90%
Fixed rate senior notes issued		$ 750
Senior Notes [Member]
Schedule Of Borrowings [Line Items]
Fixed rate senior notes issued		$ 1,500
Junior Subordinated Debt [Member]
Schedule Of Borrowings [Line Items]
Debt Maturity, Start Year	Jan. 01, 2027
Debt Maturity, End Year	Dec. 31, 2033
Junior subordinated debentures redeemed	$ 323
Trust Preferred Securities Subject to Mandatory Redemption [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures				$ 350