Loans - Unallocated Portion of Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 106,607	$ 105,708	$ 99,542	$ 98,286	$ 95,156	$ 92,438
Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	70,042		59,604
Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,508		1,680
Environmental Risk Adjustment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	14,683		13,477
Distressed Industries [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,573		3,093
Excessive Industry Concentrations [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	6,523		2,492
Large Relationship Concentrations [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	4,181		3,807
Highly-Leveraged Credit Relationships [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	7,197		5,305
Policy Exceptions [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	3,147		2,782
Credit and Collateral Exceptions [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0	3,100	2,164
Loans Not Reviewed by Concurrence [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	7,909		7,906
Adjustment for Recoveries [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	(13,071)		(15,837)
General Macroeconomic Risk [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	915		13,069
Commercial and Industrial [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	68,725	64,713	59,192	55,672	57,054	52,790
Commercial and Industrial [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	41,002		32,421
Commercial and Industrial [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,437		1,613
Commercial and Industrial [Member] | Environmental Risk Adjustment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	7,671		6,773
Commercial and Industrial [Member] | Distressed Industries [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,307		3,090
Commercial and Industrial [Member] | Excessive Industry Concentrations [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	6,151		2,114
Commercial and Industrial [Member] | Large Relationship Concentrations [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,456		2,248
Commercial and Industrial [Member] | Highly-Leveraged Credit Relationships [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	5,742		3,958
Commercial and Industrial [Member] | Policy Exceptions [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,309		1,907
Commercial and Industrial [Member] | Credit and Collateral Exceptions [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses			1,483
Commercial and Industrial [Member] | Loans Not Reviewed by Concurrence [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,044		2,110
Commercial and Industrial [Member] | Adjustment for Recoveries [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	(3,981)		(6,234)
Commercial and Industrial [Member] | General Macroeconomic Risk [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	587		7,709
Commercial Real Estate Portfolio Segment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	24,935	27,105	27,163	22,046	21,020	22,590
Commercial Real Estate Portfolio Segment [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	15,110		14,684
Commercial Real Estate Portfolio Segment [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	71		67
Commercial Real Estate Portfolio Segment [Member] | Environmental Risk Adjustment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	3,600		3,547
Commercial Real Estate Portfolio Segment [Member] | Distressed Industries [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	266		3
Commercial Real Estate Portfolio Segment [Member] | Excessive Industry Concentrations [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	372		378
Commercial Real Estate Portfolio Segment [Member] | Large Relationship Concentrations [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,725		1,559
Commercial Real Estate Portfolio Segment [Member] | Highly-Leveraged Credit Relationships [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,455		1,347
Commercial Real Estate Portfolio Segment [Member] | Policy Exceptions [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	838		875
Commercial Real Estate Portfolio Segment [Member] | Credit and Collateral Exceptions [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses			681
Commercial Real Estate Portfolio Segment [Member] | Loans Not Reviewed by Concurrence [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,224		2,284
Commercial Real Estate Portfolio Segment [Member] | Adjustment for Recoveries [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	(939)		(1,800)
Commercial Real Estate Portfolio Segment [Member] | General Macroeconomic Risk [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	213		3,538
Consumer Real Estate [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	4,702	4,907	5,178	4,659	4,533	5,230
Consumer Real Estate [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,073		2,017
Consumer Real Estate [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer Real Estate [Member] | Environmental Risk Adjustment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	469		474
Consumer Real Estate [Member] | Distressed Industries [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer Real Estate [Member] | Excessive Industry Concentrations [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer Real Estate [Member] | Large Relationship Concentrations [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer Real Estate [Member] | Highly-Leveraged Credit Relationships [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer Real Estate [Member] | Policy Exceptions [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer Real Estate [Member] | Credit and Collateral Exceptions [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses			0
Consumer Real Estate [Member] | Loans Not Reviewed by Concurrence [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,429		2,336
Consumer Real Estate [Member] | Adjustment for Recoveries [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	(311)		(364)
Consumer Real Estate [Member] | General Macroeconomic Risk [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	42		715
Consumer and Other [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	8,245	$ 8,983	8,009	6,427	$ 5,691	$ 5,010
Consumer and Other [Member] | Historical Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	11,857		10,482
Consumer and Other [Member] | Specific Valuation Allowances [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer and Other [Member] | Environmental Risk Adjustment [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	2,943		2,683
Consumer and Other [Member] | Distressed Industries [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer and Other [Member] | Excessive Industry Concentrations [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer and Other [Member] | Large Relationship Concentrations [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer and Other [Member] | Highly-Leveraged Credit Relationships [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer and Other [Member] | Policy Exceptions [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	0		0
Consumer and Other [Member] | Credit and Collateral Exceptions [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses			0
Consumer and Other [Member] | Loans Not Reviewed by Concurrence [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	1,212		1,176
Consumer and Other [Member] | Adjustment for Recoveries [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	(7,840)		(7,439)
Consumer and Other [Member] | General Macroeconomic Risk [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	73		1,107
Unallocated [Member]
Valuation Allowance for Impairment of Recognized Servicing Assets [Line Items]
Portion of allowance for loan losses	$ 0		$ 0	$ 9,482