Loans Held for Investment - Internal Risk Grading System (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Credit Risk Grades
Gross loans	$ 6,273,775	$ 6,222,053
Pass
Credit Risk Grades
Gross loans	6,208,457	6,160,136
Special Mention
Credit Risk Grades
Gross loans	14,064	13,612
Substandard
Credit Risk Grades
Gross loans	51,254	48,305
Doubtful
Credit Risk Grades
Gross loans	0	0
Business loans | Commercial and industrial
Credit Risk Grades
Gross loans	1,062,385	1,086,659
Business loans | Franchise
Credit Risk Grades
Gross loans	692,846	660,414
Business loans | Commercial owner occupied
Credit Risk Grades
Gross loans	1,284,528	1,295,215
Business loans | SBA
Credit Risk Grades
Gross loans	195,990	202,938
Business loans | Agribusiness
Credit Risk Grades
Gross loans	149,256	116,066
Business loans | Pass | Commercial and industrial
Credit Risk Grades
Gross loans	1,041,457	1,063,452
Business loans | Pass | Franchise
Credit Risk Grades
Gross loans	692,846	660,414
Business loans | Pass | Commercial owner occupied
Credit Risk Grades
Gross loans	1,262,138	1,273,381
Business loans | Pass | SBA
Credit Risk Grades
Gross loans	193,527	199,468
Business loans | Pass | Agribusiness
Credit Risk Grades
Gross loans	136,139	108,143
Business loans | Special Mention | Commercial and industrial
Credit Risk Grades
Gross loans	8,172	8,163
Business loans | Special Mention | Franchise
Credit Risk Grades
Gross loans	0	0
Business loans | Special Mention | Commercial owner occupied
Credit Risk Grades
Gross loans	4,170	654
Business loans | Special Mention | SBA
Credit Risk Grades
Gross loans	890	1
Business loans | Special Mention | Agribusiness
Credit Risk Grades
Gross loans	0	4,079
Business loans | Substandard | Commercial and industrial
Credit Risk Grades
Gross loans	12,756	15,044
Business loans | Substandard | Franchise
Credit Risk Grades
Gross loans	0	0
Business loans | Substandard | Commercial owner occupied
Credit Risk Grades
Gross loans	18,220	21,180
Business loans | Substandard | SBA
Credit Risk Grades
Gross loans	1,573	3,469
Business loans | Substandard | Agribusiness
Credit Risk Grades
Gross loans	13,117	3,844
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	1,227,693	1,243,115
Real estate loans | Multi-family
Credit Risk Grades
Gross loans	817,963	794,384
Real estate loans | One-to-four family
Credit Risk Grades
Gross loans	266,324	270,894
Real estate loans | Construction
Credit Risk Grades
Gross loans	319,610	282,811
Real estate loans | Farmland
Credit Risk Grades
Gross loans	136,522	145,393
Real estate loans | Land
Credit Risk Grades
Gross loans	34,452	31,233
Real estate loans | Pass | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,226,642	1,242,045
Real estate loans | Pass | Multi-family
Credit Risk Grades
Gross loans	816,506	794,156
Real estate loans | Pass | One-to-four family
Credit Risk Grades
Gross loans	264,574	268,776
Real estate loans | Pass | Construction
Credit Risk Grades
Gross loans	318,923	282,294
Real estate loans | Pass | Farmland
Credit Risk Grades
Gross loans	135,413	144,234
Real estate loans | Pass | Land
Credit Risk Grades
Gross loans	34,221	30,979
Real estate loans | Special Mention | Commercial non-owner occupied
Credit Risk Grades
Gross loans	0	0
Real estate loans | Special Mention | Multi-family
Credit Risk Grades
Gross loans	0	0
Real estate loans | Special Mention | One-to-four family
Credit Risk Grades
Gross loans	145	154
Real estate loans | Special Mention | Construction
Credit Risk Grades
Gross loans	687	517
Real estate loans | Special Mention | Farmland
Credit Risk Grades
Gross loans	0	44
Real estate loans | Special Mention | Land
Credit Risk Grades
Gross loans	0	0
Real estate loans | Substandard | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,051	1,070
Real estate loans | Substandard | Multi-family
Credit Risk Grades
Gross loans	1,457	228
Real estate loans | Substandard | One-to-four family
Credit Risk Grades
Gross loans	1,605	1,964
Real estate loans | Substandard | Construction
Credit Risk Grades
Gross loans	0	0
Real estate loans | Substandard | Farmland
Credit Risk Grades
Gross loans	1,109	1,115
Real estate loans | Substandard | Land
Credit Risk Grades
Gross loans	231	254
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	86,206	92,931
Consumer Loans | Pass | Consumer loans
Credit Risk Grades
Gross loans	86,071	92,794
Consumer Loans | Special Mention | Consumer loans
Credit Risk Grades
Gross loans	0	0
Consumer Loans | Substandard | Consumer loans
Credit Risk Grades
Gross loans	135	137
Consumer Loans | Doubtful | Consumer loans
Credit Risk Grades
Gross loans	$ 0	$ 0