Loans Held for Investment - Internal Risk Grading System (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Credit Risk Grades
Gross loans	$ 6,294,728	$ 6,222,053
Pass
Credit Risk Grades
Gross loans	6,239,906	6,160,136
Special Mention
Credit Risk Grades
Gross loans	12,528	13,612
Substandard
Credit Risk Grades
Gross loans	42,294	48,305
Doubtful
Credit Risk Grades
Gross loans	0	0
Business loans | Commercial and industrial
Credit Risk Grades
Gross loans	1,102,586	1,086,659
Business loans | Franchise
Credit Risk Grades
Gross loans	708,957	660,414
Business loans | Commercial owner occupied
Credit Risk Grades
Gross loans	1,310,722	1,295,215
Business loans | SBA
Credit Risk Grades
Gross loans	185,632	202,938
Business loans | Agribusiness
Credit Risk Grades
Gross loans	136,962	116,066
Business loans | Pass | Commercial and industrial
Credit Risk Grades
Gross loans	1,080,580	1,063,452
Business loans | Pass | Franchise
Credit Risk Grades
Gross loans	708,748	660,414
Business loans | Pass | Commercial owner occupied
Credit Risk Grades
Gross loans	1,296,927	1,273,381
Business loans | Pass | SBA
Credit Risk Grades
Gross loans	181,347	199,468
Business loans | Pass | Agribusiness
Credit Risk Grades
Gross loans	128,709	108,143
Business loans | Special Mention | Commercial and industrial
Credit Risk Grades
Gross loans	9,952	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	641	654
Business loans | Special Mention | SBA
Credit Risk Grades
Gross loans	1,096	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	12,054	15,044
Business loans | Substandard | Franchise
Credit Risk Grades
Gross loans	209	0
Business loans | Substandard | Commercial owner occupied
Credit Risk Grades
Gross loans	13,154	21,180
Business loans | Substandard | SBA
Credit Risk Grades
Gross loans	3,189	3,469
Business loans | Substandard | Agribusiness
Credit Risk Grades
Gross loans	8,253	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	1,224,690	1,243,115
Real estate loans | Multi-family
Credit Risk Grades
Gross loans	805,494	794,384
Real estate loans | One-to-four family
Credit Risk Grades
Gross loans	249,495	270,894
Real estate loans | Construction
Credit Risk Grades
Gross loans	321,423	282,811
Real estate loans | Farmland
Credit Risk Grades
Gross loans	136,548	145,393
Real estate loans | Land
Credit Risk Grades
Gross loans	30,246	31,233
Real estate loans | Pass | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,222,448	1,242,045
Real estate loans | Pass | Multi-family
Credit Risk Grades
Gross loans	803,977	794,156
Real estate loans | Pass | One-to-four family
Credit Risk Grades
Gross loans	247,452	268,776
Real estate loans | Pass | Construction
Credit Risk Grades
Gross loans	321,423	282,294
Real estate loans | Pass | Farmland
Credit Risk Grades
Gross loans	136,422	144,234
Real estate loans | Pass | Land
Credit Risk Grades
Gross loans	30,036	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	703	0
Real estate loans | Special Mention | One-to-four family
Credit Risk Grades
Gross loans	136	154
Real estate loans | Special Mention | Construction
Credit Risk Grades
Gross loans	0	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	2,242	1,070
Real estate loans | Substandard | Multi-family
Credit Risk Grades
Gross loans	814	228
Real estate loans | Substandard | One-to-four family
Credit Risk Grades
Gross loans	1,907	1,964
Real estate loans | Substandard | Construction
Credit Risk Grades
Gross loans	0	0
Real estate loans | Substandard | Farmland
Credit Risk Grades
Gross loans	126	1,115
Real estate loans | Substandard | Land
Credit Risk Grades
Gross loans	210	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	81,973	92,931
Consumer Loans | Pass | Consumer loans
Credit Risk Grades
Gross loans	81,837	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	136	137
Consumer Loans | Doubtful | Consumer loans
Credit Risk Grades
Gross loans	$ 0	$ 0