Loans Held for Investment - Delinquencies (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Past Due [Line Items]
Loans held for investment	$ 13,594,598	$ 13,236,433
Current
Financing Receivable, Past Due [Line Items]
Loans held for investment	13,575,263	13,223,111
30-59
Financing Receivable, Past Due [Line Items]
Loans held for investment	207	1,269
60-89
Financing Receivable, Past Due [Line Items]
Loans held for investment	83	57
90
Financing Receivable, Past Due [Line Items]
Loans held for investment	19,045	11,996
Investor loans secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	8,700,428	8,225,765
Investor loans secured by real estate | CRE non-owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,810,233	2,675,085
Investor loans secured by real estate | Multifamily
Financing Receivable, Past Due [Line Items]
Loans held for investment	5,539,464	5,171,356
Investor loans secured by real estate | Construction and land
Financing Receivable, Past Due [Line Items]
Loans held for investment	297,728	321,993
Investor loans secured by real estate | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	53,003	57,331
Investor loans secured by real estate | Current
Financing Receivable, Past Due [Line Items]
Loans held for investment	8,689,645	8,223,750
Investor loans secured by real estate | Current | CRE non-owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,799,890	2,674,328
Investor loans secured by real estate | Current | Multifamily
Financing Receivable, Past Due [Line Items]
Loans held for investment	5,539,464	5,171,355
Investor loans secured by real estate | Current | Construction and land
Financing Receivable, Past Due [Line Items]
Loans held for investment	297,728	321,993
Investor loans secured by real estate | Current | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	52,563	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	0	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	0	0
Investor loans secured by real estate | 90
Financing Receivable, Past Due [Line Items]
Loans held for investment	10,783	2,014
Investor loans secured by real estate | 90 | CRE non-owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	10,343	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	440	1,257
Business loans secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,520,343	2,541,577
Business loans secured by real estate | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	72,923	79,595
Business loans secured by real estate | CRE owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,089,300	2,114,050
Business loans secured by real estate | Franchise real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	358,120	347,932
Business loans secured by real estate | Current
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,514,877	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	72,473	78,036
Business loans secured by real estate | Current | CRE owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,084,284	2,108,746
Business loans secured by real estate | Current | Franchise real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	358,120	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	5,466	6,377
Business loans secured by real estate | 90 | SBA secured by real estate
Financing Receivable, Past Due [Line Items]
Loans held for investment	450	1,073
Business loans secured by real estate | 90 | CRE owner-occupied
Financing Receivable, Past Due [Line Items]
Loans held for investment	5,016	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	2,210,359	2,229,588
Commercial loans | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans held for investment	1,795,144	1,768,834
Commercial loans | Franchise non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	401,315	444,797
Commercial loans | SBA non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	13,900	15,957
Commercial loans | Current
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,207,451	2,225,160
Commercial loans | Current | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans held for investment	1,792,913	1,765,451
Commercial loans | Current | Franchise non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	401,315	444,797
Commercial loans | Current | SBA non-real estate secured
Financing Receivable, Past Due [Line Items]
Loans held for investment	13,223	14,912
Commercial loans | 30-59
Financing Receivable, Past Due [Line Items]
Loans held for investment	29	766
Commercial loans | 30-59 | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans held for investment	29	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	0	338
Commercial loans | 60-89
Financing Receivable, Past Due [Line Items]
Loans held for investment	83	57
Commercial loans | 60-89 | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans held for investment	83	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	0	0
Commercial loans | 90
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,796	3,605
Commercial loans | 90 | Commercial and industrial
Financing Receivable, Past Due [Line Items]
Loans held for investment	2,119	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	677	707
Retail loans
Financing Receivable, Past Due [Line Items]
Loans held for investment	163,468	239,503
Retail loans | Single family residential
Financing Receivable, Past Due [Line Items]
Loans held for investment	157,228	232,574
Retail loans | Consumer
Financing Receivable, Past Due [Line Items]
Loans held for investment	6,240	6,929
Retail loans | Current
Financing Receivable, Past Due [Line Items]
Loans held for investment	163,290	239,487
Retail loans | Current | Single family residential
Financing Receivable, Past Due [Line Items]
Loans held for investment	157,050	232,559
Retail loans | Current | Consumer
Financing Receivable, Past Due [Line Items]
Loans held for investment	6,240	6,928
Retail loans | 30-59
Financing Receivable, Past Due [Line Items]
Loans held for investment	178	16
Retail loans | 30-59 | Single family residential
Financing Receivable, Past Due [Line Items]
Loans held for investment	178	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