Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Debt Outstanding [Line Items]
Short-term borrowings	$ 7,460,000		$ 166,110,000
Total long-term debt	457,194,000		457,762,000
Federal Funds Purchased
Debt Outstanding [Line Items]
Principal value	0
Short-term borrowings	0		160,000,000
Other short-term borrowings
Debt Outstanding [Line Items]
Principal value	7,460,000	[1]
Short-term borrowings	7,460,000	[1]	6,110,000	[1]
5.375% Senior Notes
Debt Outstanding [Line Items]
Maturity	Sep. 15, 2020
Principal value	350,000,000
Senior Notes	348,048,000		347,995,000
6.05% Subordinated Notes
Debt Outstanding [Line Items]
Maturity	Jun. 01, 2017	[2]
Principal value	45,964,000.000	[2]
Subordinated Notes	53,993,000	[2]	54,571,000	[2]
7.0% Junior Subordinated Debentures
Debt Outstanding [Line Items]
Maturity	Oct. 15, 2033
Principal value	50,000,000
Junior Subordinated Debentures	$ 55,153,000		$ 55,196,000

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

[2] At March 31, 2013 and December 31, 2012, included in the carrying value of our 6.05% Subordinated Notes were $8.5 million and $9.0 million, respectively, related to hedge accounting associated with the notes.