Borrowings - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Feb. 27, 2014	Feb. 28, 2015
Schedule Of Borrowings [Line Items]
Junior subordinated debentures	$ 836,000,000
Debt Maturity, Start Year	Jan. 01, 2027
Debt Maturity, End Year	Dec. 31, 2033
Interest rate of debt instrument			8.50%
Agreements to repurchase securities	1,400,000,000	1,400,000,000
Collateral posted	1,500,000,000	1,500,000,000
Senior Notes Due Two Thousand Twenty [Member]
Schedule Of Borrowings [Line Items]
Interest rate of debt instrument				2.10%
Fixed rate senior notes issued				750,000,000
Senior Notes Due Two Thousand Twenty Five [Member]
Schedule Of Borrowings [Line Items]
Interest rate of debt instrument				2.90%
Fixed rate senior notes issued				750,000,000
Senior Notes [Member]
Schedule Of Borrowings [Line Items]
Fixed rate senior notes issued				1,500,000,000
Junior Subordinated Debt [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures redeemed	323,000,000
Trust Preferred Securities Subject to Mandatory Redemption [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures			350,000,000