Loans Receivable and Allowance for Loan Losses - Non-Covered Loans and Covered Loans by Class and Performance Status (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 7,116	$ 24,063
Non-Accrual	29,842	17,792
Current	6,999,227	6,080,136
Loans receivable	7,061,338	6,154,637
Loans receivable, gross	$ 7,063,775	6,154,561
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	$ 27,590	32,570
30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,611	21,250
90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,505	2,813
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	12,573
Non-Accrual	0	0
Current	3,617,062	3,200,322
Loans receivable, gross	3,618,989	3,214,999
Multi-family | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	1,927	2,104
Multi-family | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	12,573
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	0	350
Non-Accrual	20,423	8,443
Current	1,093,997	978,881
Loans receivable, gross	1,115,222	988,711
Commercial and industrial | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	802	1,037
Commercial and industrial | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	350
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	137
Non-Accrual	2,949	2,039
Current	472,832	379,227
Loans receivable, gross	486,567	393,632
Commercial real estate - owner occupied | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	10,786	12,229
Commercial real estate - owner occupied | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	137
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	0
Non-Accrual	184	2,057
Current	1,232,212	1,185,331
Loans receivable, gross	1,237,849	1,193,715
Commercial real estate - non-owner occupied | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	5,453	6,327
Commercial real estate - non-owner occupied | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	0
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	73,203	64,789
Loans receivable, gross	73,203	64,789
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	1,607	4,417
Non-Accrual	4,269	2,959
Current	423,551	178,559
Loans receivable, gross	435,188	193,502
Residential real estate | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	5,761	7,567
Residential real estate | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,607	4,417
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	5,442	6,574
Non-Accrual	1,959	2,236
Current	82,896	89,850
Loans receivable, gross	92,938	101,730
Manufactured housing | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	2,641	3,070
Manufactured housing | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,937	3,761
Manufactured housing | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,505	2,813
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	67	12
Non-Accrual	58	58
Current	3,474	3,177
Loans receivable, gross	3,819	3,483
Other | Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable	220	236
Other | 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	67	12
Other | 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 0	$ 0