Short-Term Borrowings - Schedule of Short-Term Borrowings and Weighted-Average Interest Rates (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Standby Letters of Credit [Line Items]
Outstanding at end of period	$ 418,971	$ 589,815	$ 452,060
Average amount outstanding	554,515
Weighted average interest rate during period	0.14%
Weighted average interest rate at end of period	0.07%
Federal Funds Purchased [Member]
Standby Letters of Credit [Line Items]
Outstanding at end of period	60,813
Average amount outstanding	218,456
Maximum amount outstanding at any month-end	466,861
Weighted average interest rate during period	0.23%
Weighted average interest rate at end of period	0.14%
Repurchase Agreements [Member]
Standby Letters of Credit [Line Items]
Outstanding at end of period	358,158
Average amount outstanding	336,059
Maximum amount outstanding at any month-end	$ 425,191
Weighted average interest rate during period	0.08%
Weighted average interest rate at end of period	0.05%