Borrowings - Amounts and Interest Rates of Short-term Borrowings (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Borrowings [Line Items]
Other Short-term borrowings		$ 50,023
Federal funds purchased and short term borrowings	1,074,482	782,082	947,432
Weighted-average interest rate	0.11%	0.09%	0.19%
Daily-average amount outstanding	838,902	826,808	1,854,033
Weighted-average interest rate	0.15%	0.12%	0.16%
Federal Funds Purchased and Repurchase Agreements [Member]
Schedule Of Borrowings [Line Items]
Federal Funds Purchased and Repurchase Agreements	1,074,482	732,059	866,555
Weighted-average interest rate	0.11%	0.04%	0.19%
Highest amount at a month-end	1,224,194	1,389,237	2,612,727
Daily-average amount outstanding	822,859	706,749	1,749,525
Weighted-average interest rate	0.15%	0.11%	0.15%
Other Short-term Borrowings [Member]
Schedule Of Borrowings [Line Items]
Other Short-term borrowings		50,023	80,877
Weighted-average interest rate		0.70%	0.20%
Highest amount at a month-end	50,016	142,927	236,842
Daily-average amount outstanding	$ 16,043	$ 120,059	$ 104,508
Weighted-average interest rate	0.57%	0.18%	0.33%