Allowance for Loan Losses and Credit Quality Information - Schedule of Commercial Credit Exposure Credit Risk Profile by Internally Assigned Risk Rating (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	100.00%	100.00%
Accrual	$ 2,266,277	$ 2,239,812
Special mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	53,223	73,729
Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	123,680	119,950
Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	20,208	22,019
Doubtful/Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	10,502	11,024
Special Mention and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	9.00%	10.00%
Accrual	207,613	226,722
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	91.00%	90.00%
Accrual	2,058,664	2,013,090
Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	732,822	704,491
Commercial [Member] | Special mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	9,091	14,611
Commercial [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	66,483	63,074
Commercial [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	1,691	1,598
Commercial [Member] | Doubtful/Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	3,300	3,263
Commercial [Member] | Special Mention and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	80,565	82,546
Commercial [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	652,257	621,945
Owner-Occupied Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	766,224	770,581
Owner-Occupied Commercial [Member] | Special mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	26,906	27,398
Owner-Occupied Commercial [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	45,129	44,899
Owner-Occupied Commercial [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	12,171	13,827
Owner-Occupied Commercial [Member] | Doubtful/Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	1,092	174
Owner-Occupied Commercial [Member] | Special Mention and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	85,298	86,298
Owner-Occupied Commercial [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	680,926	684,283
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	633,750	631,365
Commercial Mortgages [Member] | Special mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	14,908	29,267
Commercial Mortgages [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	6,078	6,222
Commercial Mortgages [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	5,130	5,422
Commercial Mortgages [Member] | Doubtful/Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	6,110	7,212
Commercial Mortgages [Member] | Special Mention and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	32,226	48,123
Commercial Mortgages [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	601,524	583,242
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	133,481	133,375
Construction [Member] | Special mention [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	2,318	2,453
Construction [Member] | Accrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	5,990	5,755
Construction [Member] | Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	1,216	1,172
Construction [Member] | Doubtful/Nonaccrual [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual		375
Construction [Member] | Special Mention and Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	9,524	9,755
Construction [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrual	$ 123,957	$ 123,620