Loans Held for Investment - Internal Risk Grading System (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Credit Risk Grades
Gross loans	$ 8,886,721	$ 8,852,005
Pass
Credit Risk Grades
Gross loans	8,818,115	8,764,593
Special Mention
Credit Risk Grades
Gross loans	17,666	25,956
Substandard
Credit Risk Grades
Gross loans	50,940	61,456
Doubtful
Credit Risk Grades
Gross loans	0	0
Business loans | Commercial and industrial
Credit Risk Grades
Gross loans	1,336,520	1,364,461
Business loans | Franchise
Credit Risk Grades
Gross loans	813,057	765,416
Business loans | Commercial owner occupied
Credit Risk Grades
Gross loans	1,648,762	1,679,122
Business loans | SBA
Credit Risk Grades
Gross loans	197,877	198,201
Business loans | Agribusiness
Credit Risk Grades
Gross loans	134,603	138,519
Business loans | Pass | Commercial and industrial
Credit Risk Grades
Gross loans	1,323,301	1,340,322
Business loans | Pass | Franchise
Credit Risk Grades
Gross loans	802,898	760,795
Business loans | Pass | Commercial owner occupied
Credit Risk Grades
Gross loans	1,634,458	1,660,994
Business loans | Pass | SBA
Credit Risk Grades
Gross loans	189,338	189,006
Business loans | Pass | Agribusiness
Credit Risk Grades
Gross loans	120,753	125,355
Business loans | Special Mention | Commercial and industrial
Credit Risk Grades
Gross loans	6,481	12,005
Business loans | Special Mention | Franchise
Credit Risk Grades
Gross loans	4,289	4,431
Business loans | Special Mention | Commercial owner occupied
Credit Risk Grades
Gross loans	948	1,580
Business loans | Special Mention | SBA
Credit Risk Grades
Gross loans	1,174	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	6,738	12,134
Business loans | Substandard | Franchise
Credit Risk Grades
Gross loans	5,870	190
Business loans | Substandard | Commercial owner occupied
Credit Risk Grades
Gross loans	13,356	16,548
Business loans | Substandard | SBA
Credit Risk Grades
Gross loans	7,365	6,906
Business loans | Substandard | Agribusiness
Credit Risk Grades
Gross loans	13,850	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	2,126,801	2,004,536
Real estate loans | Multi-family
Credit Risk Grades
Gross loans	1,511,942	1,535,289
Real estate loans | One-to-four family
Credit Risk Grades
Gross loans	279,467	356,264
Real estate loans | Construction
Credit Risk Grades
Gross loans	538,197	523,643
Real estate loans | Farmland
Credit Risk Grades
Gross loans	167,345	150,502
Real estate loans | Land
Credit Risk Grades
Gross loans	46,848	46,628
Real estate loans | Pass | Commercial non-owner occupied
Credit Risk Grades
Gross loans	2,125,422	1,998,118
Real estate loans | Pass | Multi-family
Credit Risk Grades
Gross loans	1,507,242	1,530,567
Real estate loans | Pass | One-to-four family
Credit Risk Grades
Gross loans	277,450	350,083
Real estate loans | Pass | Construction
Credit Risk Grades
Gross loans	538,197	523,643
Real estate loans | Pass | Farmland
Credit Risk Grades
Gross loans	167,228	150,381
Real estate loans | Pass | Land
Credit Risk Grades
Gross loans	46,653	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	4,039	4,060
Real estate loans | Special Mention | One-to-four family
Credit Risk Grades
Gross loans	601	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	134	132
Real estate loans | Substandard | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,379	5,687
Real estate loans | Substandard | Multi-family
Credit Risk Grades
Gross loans	661	662
Real estate loans | Substandard | One-to-four family
Credit Risk Grades
Gross loans	1,416	5,453
Real estate loans | Substandard | Construction
Credit Risk Grades
Gross loans	0	0
Real estate loans | Substandard | Farmland
Credit Risk Grades
Gross loans	117	121
Real estate loans | Substandard | Land
Credit Risk Grades
Gross loans	61	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	85,302	89,424
Consumer Loans | Pass | Consumer loans
Credit Risk Grades
Gross loans	85,175	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	127	103
Consumer Loans | Doubtful | Consumer loans
Credit Risk Grades
Gross loans	$ 0	$ 0