Loans Held for Investment - Internal Risk Grading System (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Credit Risk Grades
Gross loans	$ 8,790,435	$ 8,852,005
Pass
Credit Risk Grades
Gross loans	8,721,010	8,764,593
Special Mention
Credit Risk Grades
Gross loans	31,571	25,956
Substandard
Credit Risk Grades
Gross loans	37,854	61,456
Doubtful
Credit Risk Grades
Gross loans	0	0
Business loans | Commercial and industrial
Credit Risk Grades
Gross loans	1,300,083	1,364,461
Business loans | Franchise
Credit Risk Grades
Gross loans	860,299	765,416
Business loans | Commercial owner occupied
Credit Risk Grades
Gross loans	1,667,912	1,679,122
Business loans | SBA
Credit Risk Grades
Gross loans	187,530	198,201
Business loans | Agribusiness
Credit Risk Grades
Gross loans	126,857	138,519
Business loans | Pass | Commercial and industrial
Credit Risk Grades
Gross loans	1,287,607	1,340,322
Business loans | Pass | Franchise
Credit Risk Grades
Gross loans	845,659	760,795
Business loans | Pass | Commercial owner occupied
Credit Risk Grades
Gross loans	1,659,772	1,660,994
Business loans | Pass | SBA
Credit Risk Grades
Gross loans	179,488	189,006
Business loans | Pass | Agribusiness
Credit Risk Grades
Gross loans	112,796	125,355
Business loans | Special Mention | Commercial and industrial
Credit Risk Grades
Gross loans	5,150	12,005
Business loans | Special Mention | Franchise
Credit Risk Grades
Gross loans	13,630	4,431
Business loans | Special Mention | Commercial owner occupied
Credit Risk Grades
Gross loans	953	1,580
Business loans | Special Mention | SBA
Credit Risk Grades
Gross loans	1,131	2,289
Business loans | Special Mention | Agribusiness
Credit Risk Grades
Gross loans	765	0
Business loans | Substandard | Commercial and industrial
Credit Risk Grades
Gross loans	7,326	12,134
Business loans | Substandard | Franchise
Credit Risk Grades
Gross loans	1,010	190
Business loans | Substandard | Commercial owner occupied
Credit Risk Grades
Gross loans	7,187	16,548
Business loans | Substandard | SBA
Credit Risk Grades
Gross loans	6,911	6,906
Business loans | Substandard | Agribusiness
Credit Risk Grades
Gross loans	13,296	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	2,122,674	2,004,536
Real estate loans | Multi-family
Credit Risk Grades
Gross loans	1,520,135	1,535,289
Real estate loans | One-to-four family
Credit Risk Grades
Gross loans	248,392	356,264
Real estate loans | Construction
Credit Risk Grades
Gross loans	505,401	523,643
Real estate loans | Farmland
Credit Risk Grades
Gross loans	169,724	150,502
Real estate loans | Land
Credit Risk Grades
Gross loans	40,748	46,628
Real estate loans | Pass | Commercial non-owner occupied
Credit Risk Grades
Gross loans	2,122,431	1,998,118
Real estate loans | Pass | Multi-family
Credit Risk Grades
Gross loans	1,515,897	1,530,567
Real estate loans | Pass | One-to-four family
Credit Risk Grades
Gross loans	247,539	350,083
Real estate loans | Pass | Construction
Credit Risk Grades
Gross loans	505,401	523,643
Real estate loans | Pass | Farmland
Credit Risk Grades
Gross loans	163,687	150,381
Real estate loans | Pass | Land
Credit Risk Grades
Gross loans	40,110	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	4,019	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	5,923	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	243	5,687
Real estate loans | Substandard | Multi-family
Credit Risk Grades
Gross loans	219	662
Real estate loans | Substandard | One-to-four family
Credit Risk Grades
Gross loans	853	5,453
Real estate loans | Substandard | Construction
Credit Risk Grades
Gross loans	0	0
Real estate loans | Substandard | Farmland
Credit Risk Grades
Gross loans	114	121
Real estate loans | Substandard | Land
Credit Risk Grades
Gross loans	638	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	40,680	89,424
Consumer Loans | Pass | Consumer loans
Credit Risk Grades
Gross loans	40,623	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	57	103
Consumer Loans | Doubtful | Consumer loans
Credit Risk Grades
Gross loans	$ 0	$ 0