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, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 8,559	$ 19,813
Non-Accrual	23,641	26,415
Current	7,188,087	6,695,032
Loans receivable, gross	7,242,995	6,768,175
Loans and leases receivable, at amortized cost or lower of cost or market	$ 7,242,995	6,768,175
Delinquent period	30 days
Due days for loan payments	90 days
Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	$ 22,708	26,915
30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	6,259	17,070
90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,300	2,743
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0	4,900
Non-Accrual	1,343	0
Current	3,501,450	3,495,600
Loans receivable, gross	3,504,540	3,502,381
Multi-family \| Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	1,747	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	418	103
Non-Accrual	13,287	17,392
Current	1,271,813	1,130,831
Loans receivable, gross	1,286,090	1,149,090
Commercial and industrial \| Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	572	764
Commercial and industrial \| 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	418	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	1,298	1,453
Current	545,647	472,501
Loans receivable, gross	555,614	484,728
Commercial real estate owner occupied \| Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	8,669	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	158	160
Current	1,153,107	1,213,216
Loans receivable, gross	1,157,849	1,218,719
Commercial real estate non-owner occupied \| Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	4,584	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	95,250	85,393
Loans receivable, gross	95,250	85,393
Construction \| Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	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	2,321	7,628
Non-Accrual	5,522	5,420
Current	497,211	215,361
Loans receivable, gross	509,853	234,090
Residential real estate \| Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	4,799	5,681
Residential real estate \| 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,321	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	5,775	6,771
Non-Accrual	1,921	1,959
Current	72,777	78,946
Loans receivable, gross	82,589	90,227
Manufactured housing \| Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	2,116	2,551
Manufactured housing \| 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	3,475	4,028
Manufactured housing \| 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,300	2,743
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	45	116
Non-Accrual	112	31
Current	50,832	3,184
Loans receivable, gross	51,210	3,547
Other \| Purchased Credit Impaired Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	221	216
Other \| 30-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	45	116
Other \| 90 Days or More Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 0	$ 0

Loans Receivable and Allowance for Loan Losses - Non-Covered Loans and Covered Loans by Class and Performance Status (Detail) - USD ($)
$ in Thousands