Loans Held for Investment - Delinquencies (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Loans held for investment	$ 13,982,861	$ 13,236,433
Current
Financing Receivable, Past Due [Line Items]
Loans held for investment	13,962,683	13,223,111
30-59
Financing Receivable, Past Due [Line Items]
Loans held for investment	728	1,269
60-89
Financing Receivable, Past Due [Line Items]
Loans held for investment	936	57
90+
Financing Receivable, Past Due [Line Items]
Loans held for investment	18,514	11,996
Investor loans secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	8,870,992	8,225,765
Investor loans secured by real estate | CRE non-owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,823,065	2,675,085
Investor loans secured by real estate | Multifamily
Financing Receivable, Past Due [Line Items]
Loans held for investment	5,705,666	5,171,356
Investor loans secured by real estate | Construction and land
Financing Receivable, Past Due [Line Items]
Loans held for investment	292,815	321,993
Investor loans secured by real estate | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	49,446	57,331
Investor loans secured by real estate | Current
Financing Receivable, Past Due [Line Items]
Loans held for investment	8,859,748	8,223,750
Investor loans secured by real estate | Current | CRE non-owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,812,797	2,674,328
Investor loans secured by real estate | Current | Multifamily
Financing Receivable, Past Due [Line Items]
Loans held for investment	5,705,666	5,171,355
Investor loans secured by real estate | Current | Construction and land
Financing Receivable, Past Due [Line Items]
Loans held for investment	292,815	321,993
Investor loans secured by real estate | Current | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	48,470	56,074
Investor loans secured by real estate | 30-59
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	1
Investor loans secured by real estate | 30-59 | CRE non-owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Investor loans secured by real estate | 30-59 | Multifamily
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	1
Investor loans secured by real estate | 30-59 | Construction and land
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Investor loans secured by real estate | 30-59 | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Investor loans secured by real estate | 60-89
Financing Receivable, Past Due [Line Items]
Loans held for investment	629	0
Investor loans secured by real estate | 60-89 | CRE non-owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Investor loans secured by real estate | 60-89 | Multifamily
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Investor loans secured by real estate | 60-89 | Construction and land
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Investor loans secured by real estate | 60-89 | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	629	0
Investor loans secured by real estate | 90+
Financing Receivable, Past Due [Line Items]
Loans held for investment	10,615	2,014
Investor loans secured by real estate | 90+ | CRE non-owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	10,268	757
Investor loans secured by real estate | 90+ | Multifamily
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Investor loans secured by real estate | 90+ | Construction and land
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Investor loans secured by real estate | 90+ | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	347	1,257
Business loans secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,666,582	2,541,577
Business loans secured by real estate | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	69,937	79,595
Business loans secured by real estate | CRE owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,242,164	2,114,050
Business loans secured by real estate | Franchise real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	354,481	347,932
Business loans secured by real estate | Current
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,661,163	2,534,714
Business loans secured by real estate | Current | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	69,496	78,036
Business loans secured by real estate | Current | CRE owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,237,186	2,108,746
Business loans secured by real estate | Current | Franchise real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	354,481	347,932
Business loans secured by real estate | 30-59
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	486
Business loans secured by real estate | 30-59 | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	486
Business loans secured by real estate | 30-59 | CRE owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Business loans secured by real estate | 30-59 | Franchise real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Business loans secured by real estate | 60-89
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Business loans secured by real estate | 60-89 | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Business loans secured by real estate | 60-89 | CRE owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Business loans secured by real estate | 60-89 | Franchise real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Business loans secured by real estate | 90+
Financing Receivable, Past Due [Line Items]
Loans held for investment	5,419	6,377
Business loans secured by real estate | 90+ | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	441	1,073
Business loans secured by real estate | 90+ | CRE owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	4,978	5,304
Business loans secured by real estate | 90+ | Franchise real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Commercial loans
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,294,552	2,229,588
Commercial loans | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans held for investment	1,888,870	1,768,834
Commercial loans | Franchise non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	392,950	444,797
Commercial loans | SBA non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	12,732	15,957
Commercial loans | Current
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,291,037	2,225,160
Commercial loans | Current | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans held for investment	1,886,386	1,765,451
Commercial loans | Current | Franchise non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	392,950	444,797
Commercial loans | Current | SBA non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	11,701	14,912
Commercial loans | 30-59
Financing Receivable, Past Due [Line Items]
Loans held for investment	728	766
Commercial loans | 30-59 | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans held for investment	654	428
Commercial loans | 30-59 | Franchise non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Commercial loans | 30-59 | SBA non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	74	338
Commercial loans | 60-89
Financing Receivable, Past Due [Line Items]
Loans held for investment	307	57
Commercial loans | 60-89 | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans held for investment	14	57
Commercial loans | 60-89 | Franchise non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Commercial loans | 60-89 | SBA non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	293	0
Commercial loans | 90+
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,480	3,605
Commercial loans | 90+ | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans held for investment	1,816	2,898
Commercial loans | 90+ | Franchise non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Commercial loans | 90+ | SBA non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	664	707
Retail loans
Financing Receivable, Past Due [Line Items]
Loans held for investment	150,735	239,503
Retail loans | Single family residential
Financing Receivable, Past Due [Line Items]
Loans held for investment	144,309	232,574
Retail loans | Consumer
Financing Receivable, Past Due [Line Items]
Loans held for investment	6,426	6,929
Retail loans | Current
Financing Receivable, Past Due [Line Items]
Loans held for investment	150,735	239,487
Retail loans | Current | Single family residential
Financing Receivable, Past Due [Line Items]
Loans held for investment	144,309	232,559
Retail loans | Current | Consumer
Financing Receivable, Past Due [Line Items]
Loans held for investment	6,426	6,928
Retail loans | 30-59
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	16
Retail loans | 30-59 | Single family residential
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	15
Retail loans | 30-59 | Consumer
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	1
Retail loans | 60-89
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Retail loans | 60-89 | Single family residential
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Retail loans | 60-89 | Consumer
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Retail loans | 90+
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Retail loans | 90+ | Single family residential
Financing Receivable, Past Due [Line Items]
Loans held for investment	0	0
Retail loans | 90+ | Consumer
Financing Receivable, Past Due [Line Items]
Loans held for investment	$ 0	$ 0