Borrowings - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2014	Feb. 27, 2014	Dec. 31, 2013
Schedule Of Borrowings [Line Items]
Junior subordinated debentures	$ 835,000,000
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		1,600,000,000
2015 [Member]
Schedule Of Borrowings [Line Items]
Trust preferred capital securities	25.00%
2016 [Member]
Schedule Of Borrowings [Line Items]
Trust preferred capital securities	0.00%
Trust Preferred Securities Subject to Mandatory Redemption [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures		$ 350,000,000