Loans Receivable and Allowance for Loan Losses - Schedule of Loans Receivable (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2015 USD ($) FDIC	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	$ 4,492,206		$ 4,269,480
Total loans receivable covered under FDIC loss sharing agreements	31,689		42,181
Total loans receivable	4,523,895		4,311,661
Deferred (fees) costs and unamortized premiums/(discounts), net	930		512
Allowance for loan losses	(37,491)	$ (33,566)	(30,932)	$ (28,186)	$ (26,704)	$ (23,998)
Total loans receivable, net of allowance for loan losses	$ 4,487,334		4,281,241
Loans acquired in FDIC-assisted transactions | FDIC	2
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	$ 2,238,487		2,208,405
Allowance for loan losses	(8,734)	(8,196)	(8,493)	(7,205)	(6,220)	(4,227)
Commercial and industrial (including owner occupied commercial real estate)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	653,417		549,935
Allowance for loan losses	(14,062)	(6,747)	(4,784)	(3,341)	(2,483)	(2,674)
Commercial real estate non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	895,780		839,310
Allowance for loan losses	(6,310)	(9,738)	(9,198)	(10,060)	(9,862)	(8,961)
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	68,742		49,718
Allowance for loan losses	(844)	(852)	(1,047)	(2,187)	(2,342)	(2,385)
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	276,655		297,395
Allowance for loan losses	(3,455)	(2,995)	(2,698)	(2,028)	(2,307)	(2,490)
Manufactured housing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	119,786		126,731
Allowance for loan losses	(316)	(346)	(262)	(401)	(593)	(614)
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	3,962		4,433
Allowance for loan losses	(119)	$ (109)	(114)	$ (138)	$ (127)	$ (130)
Commercial Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	4,105,710		3,856,205
Total loans receivable covered under FDIC loss sharing agreements	17,782		26,897
Commercial Loan | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	2,236,757		2,206,403
Total loans receivable covered under FDIC loss sharing agreements	1,730		2,002
Commercial Loan | Commercial and industrial (including owner occupied commercial real estate)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	913,898		770,220
Total loans receivable covered under FDIC loss sharing agreements	6,585		2,543
Commercial Loan | Commercial real estate non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	886,982		827,940
Total loans receivable covered under FDIC loss sharing agreements	8,798		11,370
Commercial Loan | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	68,073		44,642
Total loans receivable covered under FDIC loss sharing agreements	669		5,076
Consumer Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	386,496		413,275
Total loans receivable covered under FDIC loss sharing agreements	13,907		15,284
Consumer Loan | Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	265,469		285,003
Total loans receivable covered under FDIC loss sharing agreements	11,186		12,392
Consumer Loan | Manufactured housing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	119,786		126,731
Consumer Loan | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	1,241		1,541
Total loans receivable covered under FDIC loss sharing agreements	$ 2,721		$ 2,892