Borrowings - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Schedule Of Borrowings [Line Items]
Junior subordinated debentures face value	$ 1,200,000,000
Junior subordinated debentures	1,200,000,000	1,200,000,000
Junior Subordinated Debentures Maturity Date	Jan. 31, 2068
Debt Maturity, Start Year	Jan. 01, 2027
Debt Maturity, End Year	Dec. 31, 2068
Rate on related enhanced trust preferred securities	8.50%
End date of restrictions included in the replacement capital covenant	Jan. 31, 2048
Period of days prior to redemption or repurchase of enhanced trust preferred securities applicable to provisions of the replacement capital covenant	180 days
Junior Subordinated Debenture due January 31, 2068 [Member]
Schedule Of Borrowings [Line Items]
Junior subordinated debentures	$ 350,000,000