Borrowed Funds - Additional Information (Detail) (USD $)	1 Months Ended	6 Months Ended	12 Months Ended
	Jan. 31, 2012 Y	Jun. 30, 2012 Y	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Weighted average interest rate for short-term borrowings			0.26%	0.25%
Interest expense recorded on FHLB advances			$ 79,500,000	$ 86,600,000	$ 91,500,000
Callable FHLB advances with scheduled maturities beyond 2017			408,000,000
Callable FHLB advances interest rates range, low			2.27%
Callable FHLB advances interest rates range, high			4.09%
FHLB advances modification amount	50,000,000	200,000,000
Extension of FHLB maturity dates, years	10	10
FHLB advances non-callable range high, years	4	4
Decline in interest rate on FHLB borrowings	0.94%	0.59%
Weighted average interest rate on FHLB borrowings	3.70%	3.86%
Interest expense on securities sold under repurchase agreements			24,600,000	25,000,000	28,300,000
Subordinated notes, interest rate			5.00%
Subordinated notes, due date			Jul. 15, 2015
Fair value of securities			1,200,000,000	1,500,000,000
Long-Term Borrowings [Member]
Debt Instrument [Line Items]
Securities sold under agreements to repurchase			587,500,000	587,500,000
Subordinated notes			100,000,000	100,000,000
FHLB [Member]
Debt Instrument [Line Items]
Weighted average interest rate for long-term borrowings			3.86%	3.91%
Short-term borrowings [Member]
Debt Instrument [Line Items]
Securities sold under agreements to repurchase			$ 154,300,000	$ 212,800,000
Securities Sold under Agreements to Repurchase [Member]
Debt Instrument [Line Items]
Weighted average interest rate for long-term borrowings			4.07%	4.18%