Loans Held for Investment - Internal Risk Grading System (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Credit Risk Grades
Gross loans	$ 8,767,016	$ 8,846,286
Pass
Credit Risk Grades
Gross loans	8,706,847	8,758,874
Special Mention
Credit Risk Grades
Gross loans	21,015	25,956
Substandard
Credit Risk Grades
Gross loans	39,154	61,456
Doubtful
Credit Risk Grades
Gross loans	0	0
Business loans | Commercial and industrial
Credit Risk Grades
Gross loans	1,233,938	1,364,423
Business loans | Franchise
Credit Risk Grades
Gross loans	894,023	765,416
Business loans | Commercial owner occupied
Credit Risk Grades
Gross loans	1,678,888	1,679,122
Business loans | SBA
Credit Risk Grades
Gross loans	179,965	193,882
Business loans | Agribusiness
Credit Risk Grades
Gross loans	119,633	138,519
Business loans | Pass | Commercial and industrial
Credit Risk Grades
Gross loans	1,219,358	1,340,284
Business loans | Pass | Franchise
Credit Risk Grades
Gross loans	880,632	760,795
Business loans | Pass | Commercial owner occupied
Credit Risk Grades
Gross loans	1,669,152	1,660,994
Business loans | Pass | SBA
Credit Risk Grades
Gross loans	171,393	184,687
Business loans | Pass | Agribusiness
Credit Risk Grades
Gross loans	107,551	125,355
Business loans | Special Mention | Commercial and industrial
Credit Risk Grades
Gross loans	4,438	12,005
Business loans | Special Mention | Franchise
Credit Risk Grades
Gross loans	13,375	4,431
Business loans | Special Mention | Commercial owner occupied
Credit Risk Grades
Gross loans	1,321	1,580
Business loans | Special Mention | SBA
Credit Risk Grades
Gross loans	1,881	2,289
Business loans | Special Mention | Agribusiness
Credit Risk Grades
Gross loans	0	0
Business loans | Substandard | Commercial and industrial
Credit Risk Grades
Gross loans	10,142	12,134
Business loans | Substandard | Franchise
Credit Risk Grades
Gross loans	16	190
Business loans | Substandard | Commercial owner occupied
Credit Risk Grades
Gross loans	8,415	16,548
Business loans | Substandard | SBA
Credit Risk Grades
Gross loans	6,691	6,906
Business loans | Substandard | Agribusiness
Credit Risk Grades
Gross loans	12,082	13,164
Business loans | Doubtful | Commercial and industrial
Credit Risk Grades
Gross loans	0	0
Business loans | Doubtful | Franchise
Credit Risk Grades
Gross loans	0	0
Business loans | Doubtful | Commercial owner occupied
Credit Risk Grades
Gross loans	0	0
Business loans | Doubtful | SBA
Credit Risk Grades
Gross loans	0	0
Business loans | Doubtful | Agribusiness
Credit Risk Grades
Gross loans	0	0
Real estate loans | Commercial non-owner occupied
Credit Risk Grades
Gross loans	2,053,590	2,003,174
Real estate loans | Multi-family
Credit Risk Grades
Gross loans	1,611,904	1,535,289
Real estate loans | One-to-four family
Credit Risk Grades
Gross loans	273,182	356,264
Real estate loans | Construction
Credit Risk Grades
Gross loans	478,961	523,643
Real estate loans | Farmland
Credit Risk Grades
Gross loans	171,667	150,502
Real estate loans | Land
Credit Risk Grades
Gross loans	30,717	46,628
Real estate loans | Pass | Commercial non-owner occupied
Credit Risk Grades
Gross loans	2,052,813	1,996,756
Real estate loans | Pass | Multi-family
Credit Risk Grades
Gross loans	1,611,686	1,530,567
Real estate loans | Pass | One-to-four family
Credit Risk Grades
Gross loans	272,555	350,083
Real estate loans | Pass | Construction
Credit Risk Grades
Gross loans	478,961	523,643
Real estate loans | Pass | Farmland
Credit Risk Grades
Gross loans	171,667	150,381
Real estate loans | Pass | Land
Credit Risk Grades
Gross loans	30,585	46,008
Real estate loans | Special Mention | Commercial non-owner occupied
Credit Risk Grades
Gross loans	0	731
Real estate loans | Special Mention | Multi-family
Credit Risk Grades
Gross loans	0	4,060
Real estate loans | Special Mention | One-to-four family
Credit Risk Grades
Gross loans	0	728
Real estate loans | Special Mention | Construction
Credit Risk Grades
Gross loans	0	0
Real estate loans | Special Mention | Farmland
Credit Risk Grades
Gross loans	0	0
Real estate loans | Special Mention | Land
Credit Risk Grades
Gross loans	0	132
Real estate loans | Substandard | Commercial non-owner occupied
Credit Risk Grades
Gross loans	777	5,687
Real estate loans | Substandard | Multi-family
Credit Risk Grades
Gross loans	218	662
Real estate loans | Substandard | One-to-four family
Credit Risk Grades
Gross loans	627	5,453
Real estate loans | Substandard | Construction
Credit Risk Grades
Gross loans	0	0
Real estate loans | Substandard | Farmland
Credit Risk Grades
Gross loans	0	121
Real estate loans | Substandard | Land
Credit Risk Grades
Gross loans	132	488
Real estate loans | Doubtful | Commercial non-owner occupied
Credit Risk Grades
Gross loans	0	0
Real estate loans | Doubtful | Multi-family
Credit Risk Grades
Gross loans	0	0
Real estate loans | Doubtful | One-to-four family
Credit Risk Grades
Gross loans	0	0
Real estate loans | Doubtful | Construction
Credit Risk Grades
Gross loans	0	0
Real estate loans | Doubtful | Farmland
Credit Risk Grades
Gross loans	0	0
Real estate loans | Doubtful | Land
Credit Risk Grades
Gross loans	0	0
Consumer loans | Consumer loans
Credit Risk Grades
Gross loans	40,548	89,424
Consumer loans | Pass | Consumer loans
Credit Risk Grades
Gross loans	40,494	89,321
Consumer loans | Special Mention | Consumer loans
Credit Risk Grades
Gross loans	0	0
Consumer loans | Substandard | Consumer loans
Credit Risk Grades
Gross loans	54	103
Consumer loans | Doubtful | Consumer loans
Credit Risk Grades
Gross loans	$ 0	$ 0