Allowance for Credit Losses - Summary of Activity in Allowance for Loan Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 130,200	$ 133,802
Loans charged-off	(11,719)	(8,546)
Charged-off loans recovered	1,819	2,240
Net charge-offs	(9,900)	(6,306)
Provision for loan losses	1,544	5,374
Ending balance	121,844	132,870
Commercial and industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	64,370	73,649
Loans charged-off	(7,325)	(4,807)
Charged-off loans recovered	1,338	1,005
Net charge-offs	(5,987)	(3,802)
Provision for loan losses	(2,651)	3,464
Ending balance	55,732	73,311
Commercial real estate loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	44,069	34,637
Loans charged-off	(1,993)	(1,080)
Charged-off loans recovered	15	120
Net charge-offs	(1,978)	(960)
Provision for loan losses	2,104	738
Ending balance	44,195	34,415
Residential mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	9,423	9,120
Loans charged-off	(892)	(1,176)
Charged-off loans recovered	70	514
Net charge-offs	(822)	(662)
Provision for loan losses	730	1,379
Ending balance	9,331	9,837
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	5,542	8,677
Loans charged-off	(1,509)	(1,483)
Charged-off loans recovered	396	601
Net charge-offs	(1,113)	(882)
Provision for loan losses	1,031	145
Ending balance	5,460	7,940
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,796	7,719
Loans charged-off
Charged-off loans recovered
Net charge-offs
Provision for loan losses	330	(352)
Ending balance	$ 7,126	$ 7,367