Loans Held for Investment - Delinquencies (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Other information concerning the credit quality
Total Gross Loans	$ 8,767,016	$ 8,846,286
Non-accruing	8,105	4,857
Current
Other information concerning the credit quality
Current	8,755,786	8,833,433
30-59
Other information concerning the credit quality
Days Past Due	1,725	7,046
60-89
Other information concerning the credit quality
Days Past Due	3,212	1,242
90
Other information concerning the credit quality
Days Past Due	6,293	4,565
Business loans | Commercial and industrial
Other information concerning the credit quality
Total Gross Loans	1,233,938	1,364,423
Non-accruing	2,950	931
Business loans | Franchise
Other information concerning the credit quality
Total Gross Loans	894,023	765,416
Non-accruing	16	190
Business loans | Commercial owner occupied
Other information concerning the credit quality
Total Gross Loans	1,678,888	1,679,122
Non-accruing	1,405	599
Business loans | SBA
Other information concerning the credit quality
Total Gross Loans	179,965	193,882
Non-accruing	2,586	2,739
Business loans | Agribusiness
Other information concerning the credit quality
Total Gross Loans	119,633	138,519
Non-accruing	0	0
Business loans | Current | Commercial and industrial
Other information concerning the credit quality
Current	1,229,223	1,361,979
Business loans | Current | Franchise
Other information concerning the credit quality
Current	893,999	759,546
Business loans | Current | Commercial owner occupied
Other information concerning the credit quality
Current	1,677,101	1,677,967
Business loans | Current | SBA
Other information concerning the credit quality
Current	176,541	190,732
Business loans | Current | Agribusiness
Other information concerning the credit quality
Current	119,633	138,519
Business loans | 30-59 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	101	309
Business loans | 30-59 | Franchise
Other information concerning the credit quality
Days Past Due	8	5,680
Business loans | 30-59 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	382	343
Business loans | 30-59 | SBA
Other information concerning the credit quality
Days Past Due	731	524
Business loans | 30-59 | Agribusiness
Other information concerning the credit quality
Days Past Due	0	0
Business loans | 60-89 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	3,105	1,204
Business loans | 60-89 | Franchise
Other information concerning the credit quality
Days Past Due	0	0
Business loans | 60-89 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	0	0
Business loans | 60-89 | SBA
Other information concerning the credit quality
Days Past Due	107	0
Business loans | 60-89 | Agribusiness
Other information concerning the credit quality
Days Past Due	0	0
Business loans | 90 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	1,509	931
Business loans | 90 | Franchise
Other information concerning the credit quality
Days Past Due	16	190
Business loans | 90 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	1,405	812
Business loans | 90 | SBA
Other information concerning the credit quality
Days Past Due	2,586	2,626
Business loans | 90 | Agribusiness
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | Commercial non-owner occupied
Other information concerning the credit quality
Total Gross Loans	2,053,590	2,003,174
Non-accruing	777	0
Real estate loans | Multi-family
Other information concerning the credit quality
Total Gross Loans	1,611,904	1,535,289
Non-accruing	0	0
Real estate loans | One-to-four family
Other information concerning the credit quality
Total Gross Loans	273,182	356,264
Non-accruing	371	398
Real estate loans | Construction
Other information concerning the credit quality
Total Gross Loans	478,961	523,643
Non-accruing	0	0
Real estate loans | Farmland
Other information concerning the credit quality
Total Gross Loans	171,667	150,502
Non-accruing	0	0
Real estate loans | Land
Other information concerning the credit quality
Total Gross Loans	30,717	46,628
Non-accruing	0	0
Real estate loans | Current | Commercial non-owner occupied
Other information concerning the credit quality
Current	2,052,813	2,003,174
Real estate loans | Current | Multi-family
Other information concerning the credit quality
Current	1,611,904	1,535,275
Real estate loans | Current | One-to-four family
Other information concerning the credit quality
Current	272,679	356,219
Real estate loans | Current | Construction
Other information concerning the credit quality
Current	478,961	523,643
Real estate loans | Current | Farmland
Other information concerning the credit quality
Current	171,667	150,502
Real estate loans | Current | Land
Other information concerning the credit quality
Current	30,717	46,628
Real estate loans | 30-59 | Commercial non-owner occupied
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 30-59 | Multi-family
Other information concerning the credit quality
Days Past Due	0	14
Real estate loans | 30-59 | One-to-four family
Other information concerning the credit quality
Days Past Due	503	30
Real estate loans | 30-59 | Construction
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 30-59 | Farmland
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 30-59 | Land
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 60-89 | Commercial non-owner occupied
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 60-89 | Multi-family
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 60-89 | One-to-four family
Other information concerning the credit quality
Days Past Due	0	9
Real estate loans | 60-89 | Construction
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 60-89 | Farmland
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 60-89 | Land
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 90 | Commercial non-owner occupied
Other information concerning the credit quality
Days Past Due	777	0
Real estate loans | 90 | Multi-family
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 90 | One-to-four family
Other information concerning the credit quality
Days Past Due	0	6
Real estate loans | 90 | Construction
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 90 | Farmland
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 90 | Land
Other information concerning the credit quality
Days Past Due	0	0
Consumer loans | Consumer loans
Other information concerning the credit quality
Total Gross Loans	40,548	89,424
Non-accruing	0	0
Consumer loans | Current | Consumer loans
Other information concerning the credit quality
Current	40,548	89,249
Consumer loans | 30-59 | Consumer loans
Other information concerning the credit quality
Days Past Due	0	146
Consumer loans | 60-89 | Consumer loans
Other information concerning the credit quality
Days Past Due	0	29
Consumer loans | 90 | Consumer loans
Other information concerning the credit quality
Days Past Due	$ 0	$ 0