Loans Receivable and Allowance for Loan Losses - Non-Covered Loans and Covered Loans by Class and Performance Status (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 10,884	$ 19,813
Non-Accrual	23,191	26,415
Current	6,884,094	6,695,032
Loans receivable	6,943,566	6,768,258
Loans receivable, gross	$ 6,944,266	6,768,175
Due days for loan payments	30 days
Delinquent period	90 days
Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	$ 26,097	26,915
30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	8,138	17,070
90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,746	2,743
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	4,900
Non-Accrual	0	0
Current	3,643,539	3,495,600
Loans receivable, gross	3,645,374	3,502,381
Multi-family | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,835	1,881
Multi-family | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	4,900
Multi-family | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	129	103
Non-Accrual	14,220	17,392
Current	1,187,571	1,130,831
Loans receivable, gross	1,202,641	1,149,090
Commercial and industrial | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	721	764
Commercial and industrial | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	129	103
Commercial and industrial | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Commercial real estate - owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	202
Non-Accrual	1,437	1,453
Current	490,277	472,501
Loans receivable, gross	502,150	484,728
Commercial real estate - owner occupied | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	10,436	10,572
Commercial real estate - owner occupied | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	202
Commercial real estate - owner occupied | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Commercial real estate - non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	93
Non-Accrual	242	160
Current	1,190,591	1,213,216
Loans receivable, gross	1,195,904	1,218,719
Commercial real estate - non-owner occupied | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	5,071	5,250
Commercial real estate - non-owner occupied | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	93
Commercial real estate - non-owner occupied | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Non-Accrual	0	0
Current	81,101	85,393
Loans receivable, gross	81,101	85,393
Construction | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	0	0
Construction | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Construction | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,490	7,628
Non-Accrual	5,216	5,420
Current	210,825	215,361
Loans receivable, gross	225,839	234,090
Residential real estate | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	5,308	5,681
Residential real estate | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,490	7,628
Residential real estate | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
Manufactured housing
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	6,190	6,771
Non-Accrual	1,979	1,959
Current	77,042	78,946
Loans receivable, gross	87,687	90,227
Manufactured housing | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,476	2,551
Manufactured housing | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	3,444	4,028
Manufactured housing | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,746	2,743
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	75	116
Non-Accrual	97	31
Current	3,148	3,184
Loans receivable, gross	3,570	3,547
Other | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	250	216
Other | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	75	116
Other | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 0	$ 0