Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012
Debt Outstanding [Line Items]
Borrowings	$ 5,747		$ 5,788	$ 17,358		$ 18,414
Short-term borrowings	5,580			5,580			166,110
Total long-term debt	455,744			455,744			457,762
Federal Funds Purchased
Debt Outstanding [Line Items]
Principal value	0			0
Short-term borrowings	0			0			160,000
Other short-term borrowings
Debt Outstanding [Line Items]
Principal value	5,580	[1]		5,580	[1]
Short-term borrowings	5,580	[1]		5,580	[1]		6,110	[1]
5.375% Senior Notes
Debt Outstanding [Line Items]
Maturity				Sep. 15, 2020
Principal value	350,000			350,000
Senior Notes	348,155			348,155			347,995
6.05% Subordinated Notes
Debt Outstanding [Line Items]
Maturity				Jun. 01, 2017	[2]
Principal value	45,964	[2]		45,964	[2]
Subordinated Notes	52,524	[2]		52,524	[2]		54,571	[2]
7.0% Junior Subordinated Debentures
Debt Outstanding [Line Items]
Maturity				Oct. 15, 2033
Principal value	50,000			50,000
Junior Subordinated Debentures	$ 55,065			$ 55,065			$ 55,196

[1] Represents cash collateral received from our counterparty for our interest rate swap agreement related to our 6.05% Subordinated Notes.

[2] At September 30, 2013 and December 31, 2012, included in the carrying value of our 6.05% Subordinated Notes were $7.0 million and $9.0 million, respectively, related to hedge accounting associated with the notes.