Short-Term Borrowings - Schedule of Short-Term Borrowings and Weighted-Average Interest Rates (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Standby Letters of Credit [Line Items]
Outstanding at end of period	$ 530,381	$ 589,815	$ 346,000
Average amount outstanding	621,237
Weighted average interest rate during period	0.16%
Weighted average interest rate at end of period	0.27%
Federal Funds Purchased [Member]
Standby Letters of Credit [Line Items]
Outstanding at end of period	105,190
Average amount outstanding	288,247
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	0.21%
Repurchase Agreements [Member]
Standby Letters of Credit [Line Items]
Outstanding at end of period	425,191
Average amount outstanding	332,990
Maximum amount outstanding at any month-end	$ 425,191
Weighted average interest rate during period	0.09%
Weighted average interest rate at end of period	0.28%