Loans Receivable and Allowance for Loan Losses - Schedule of Loans Receivable (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($) FDIC	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	$ 4,754,730		$ 4,269,480
Total loans receivable covered under FDIC loss sharing agreements	13,849		42,181
Total loans receivable	4,768,579		4,311,661
Deferred costs and unamortized premiums, net	523		512
Allowance for loan losses	(33,823)	$ (37,491)	(30,932)	$ (31,083)	$ (28,186)	$ (23,998)
Total loans receivable, net of allowance for loan losses	$ 4,735,279		4,281,241
Loans acquired in FDIC-assisted transactions | FDIC	2
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	$ 2,405,400		2,208,405
Allowance for loan losses	(9,206)	(8,734)	(8,493)	(7,972)	(7,205)	(4,227)
Commercial and industrial (including owner occupied commercial real estate)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	683,940		549,935
Allowance for loan losses	(10,429)	(14,062)	(4,784)	(4,504)	(3,341)	(2,674)
Commercial real estate non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	912,971		839,310
Allowance for loan losses	(6,119)	(6,310)	(9,198)	(11,199)	(10,060)	(8,961)
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	89,616		49,718
Allowance for loan losses	(1,110)	(844)	(1,047)	(1,699)	(2,187)	(2,385)
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	272,148		297,395
Allowance for loan losses	(3,194)	(3,455)	(2,698)	(2,129)	(2,028)	(2,490)
Manufactured housing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	116,742		126,731
Allowance for loan losses	(386)	(316)	(262)	(321)	(401)	(614)
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable	3,744		4,433
Allowance for loan losses	(120)	$ (119)	(114)	$ (111)	$ (138)	$ (130)
Commercial Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	4,375,945		3,856,205
Total loans receivable covered under FDIC loss sharing agreements	0		26,897
Commercial Loan | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	2,405,400		2,206,403
Total loans receivable covered under FDIC loss sharing agreements	0		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	967,958		777,220
Total loans receivable covered under FDIC loss sharing agreements	0		8,449
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	912,971		827,940
Total loans receivable covered under FDIC loss sharing agreements	0		11,370
Commercial Loan | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	89,616		44,642
Total loans receivable covered under FDIC loss sharing agreements	0		5,076
Consumer Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	378,785		413,275
Total loans receivable covered under FDIC loss sharing agreements	13,849		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	260,967		285,003
Total loans receivable covered under FDIC loss sharing agreements	11,181		12,392
Consumer Loan | Manufactured housing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	116,742		126,731
Consumer Loan | Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans receivable not covered under FDIC loss sharing agreements	1,076		1,541
Total loans receivable covered under FDIC loss sharing agreements	$ 2,668		$ 2,892