Short-term Borrowings - Summary of Short-Term Borrowings (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Short-term Debt [Line Items]
Ending Balance	$ 356,227	$ 312,777	$ 187,861
Average Balance	402,196	182,864	488,078
Average Rate	0.27%	0.40%	0.40%
Federal funds purchased [Member]
Short-term Debt [Line Items]
Ending Balance	34,000	75,300	12,800
Average Balance	47,727	15,642	40,322
Average Rate	0.27%	0.26%	0.26%
Borrowings from FHLB [Member]
Short-term Debt [Line Items]
Ending Balance	178,100	84,000	0
Average Balance	214,703	7,537	277,329
Average Rate	0.25%	0.21%	0.39%
Securities sold under agreements to repurchase [Member]
Short-term Debt [Line Items]
Ending Balance	144,127	153,477	170,563
Average Balance	139,766	155,551	165,945
Average Rate	0.28%	0.43%	0.47%
Treasury, tax and loan note option [Member]
Short-term Debt [Line Items]
Ending Balance	0	0	4,498
Average Balance	0	4,134	4,482
Average Rate	0.00%	0.00%	0.00%
Weighted average [Member]
Short-term Debt [Line Items]
Average Rate	0.25%	0.28%	0.43%
Maximum [Member]
Short-term Debt [Line Items]
Ending Balance	$ 486,144	$ 312,777	$ 1,000,753