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			6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012	Dec. 31, 2012
Debt Outstanding [Line Items]
Borrowings	$ 5,817		$ 6,270	$ 11,611		$ 12,626
Short-term borrowings	5,400			5,400			166,110
Total long-term debt	455,938			455,938			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	5,400	[1]		5,400	[1]
Short-term borrowings	5,400	[1]		5,400	[1]		6,110	[1]
5.375% Senior Notes
Debt Outstanding [Line Items]
Maturity				Sep. 15, 2020
Principal value	350,000			350,000
Senior Notes	348,101			348,101			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	52,728	[2]		52,728	[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	$ 55,109			$ 55,109			$ 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 June 30, 2013 and December 31, 2012, included in the carrying value of our 6.05% Subordinated Notes were $7.2 million and $9.0 million, respectively, related to hedge accounting associated with the notes.