Loans Receivable and Allowance for Loan Losses - Credit Ratings of Covered and Non-Covered Loan Portfolio (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 7,181,084	$ 6,768,175
Pass/Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	6,405,308	6,300,595
Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	117,075	99,279
Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	76,277	40,437
Performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	566,841	305,939
Non-performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	15,583	21,925
Multi-family
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	3,542,770	3,502,381
Multi-family | Pass/Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	3,485,669	3,438,554
Multi-family | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	31,001	53,873
Multi-family | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	26,100	9,954
Multi-family | Performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Multi-family | Non-performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,266,520	1,149,090
Commercial and industrial | Pass/Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,211,934	1,118,889
Commercial and industrial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	16,979	7,652
Commercial and industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	37,607	22,549
Commercial and industrial | Performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial and industrial | Non-performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial real estate - owner occupied
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	545,231	484,728
Commercial real estate - owner occupied | Pass/Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	529,898	471,826
Commercial real estate - owner occupied | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	8,152	5,987
Commercial real estate - owner occupied | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	7,181	6,915
Commercial real estate - owner occupied | Performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial real estate - owner occupied | Non-performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial real estate - non-owner occupied
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,155,998	1,218,719
Commercial real estate - non-owner occupied | Pass/Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,089,666	1,185,933
Commercial real estate - non-owner occupied | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	60,943	31,767
Commercial real estate - non-owner occupied | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	5,389	1,019
Commercial real estate - non-owner occupied | Performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Commercial real estate - non-owner occupied | Non-performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	88,141	85,393
Construction | Pass/Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	88,141	85,393
Construction | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction | Performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Construction | Non-performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	493,222	234,090
Residential real estate | Pass/Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Residential real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Residential real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Residential real estate | Performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	485,442	221,042
Residential real estate | Non-performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	7,780	13,048
Manufactured housing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	85,328	90,227
Manufactured housing | Pass/Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Manufactured housing | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Manufactured housing | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Manufactured housing | Performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	77,675	81,497
Manufactured housing | Non-performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	7,653	8,730
Other
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	3,874	3,547
Other | Pass/Satisfactory
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Other | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Other | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Other | Performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	3,724	3,400
Other | Non-performing
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 150	$ 147