Loans Held for Investment - Internal Risk Grading System (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Credit Risk Grades
Gross loans	$ 8,820,732	$ 6,222,053
Pass
Credit Risk Grades
Gross loans	8,727,280	6,160,136
Special Mention
Credit Risk Grades
Gross loans	18,772	13,612
Substandard
Credit Risk Grades
Gross loans	74,680	48,305
Doubtful
Credit Risk Grades
Gross loans	0	0
Business loans | Commercial and industrial
Credit Risk Grades
Gross loans	1,359,841	1,086,659
Business loans | Franchise
Credit Risk Grades
Gross loans	735,366	660,414
Business loans | Commercial owner occupied
Credit Risk Grades
Gross loans	1,717,150	1,295,215
Business loans | SBA
Credit Risk Grades
Gross loans	203,229	202,938
Business loans | Agribusiness
Credit Risk Grades
Gross loans	133,241	116,066
Business loans | Pass | Commercial and industrial
Credit Risk Grades
Gross loans	1,325,912	1,063,452
Business loans | Pass | Franchise
Credit Risk Grades
Gross loans	735,157	660,414
Business loans | Pass | Commercial owner occupied
Credit Risk Grades
Gross loans	1,690,912	1,273,381
Business loans | Pass | SBA
Credit Risk Grades
Gross loans	195,527	199,468
Business loans | Pass | Agribusiness
Credit Risk Grades
Gross loans	126,695	108,143
Business loans | Special Mention | Commercial and industrial
Credit Risk Grades
Gross loans	14,380	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	1,348	654
Business loans | Special Mention | SBA
Credit Risk Grades
Gross loans	1,810	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	19,549	15,044
Business loans | Substandard | Franchise
Credit Risk Grades
Gross loans	209	0
Business loans | Substandard | Commercial owner occupied
Credit Risk Grades
Gross loans	24,890	21,180
Business loans | Substandard | SBA
Credit Risk Grades
Gross loans	5,892	3,469
Business loans | Substandard | Agribusiness
Credit Risk Grades
Gross loans	6,546	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	1,932,681	1,243,115
Real estate loans | Multi-family
Credit Risk Grades
Gross loans	1,554,692	794,384
Real estate loans | One-to-four family
Credit Risk Grades
Gross loans	376,617	270,894
Real estate loans | Construction
Credit Risk Grades
Gross loans	504,708	282,811
Real estate loans | Farmland
Credit Risk Grades
Gross loans	138,479	145,393
Real estate loans | Land
Credit Risk Grades
Gross loans	49,992	31,233
Real estate loans | Pass | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,924,416	1,242,045
Real estate loans | Pass | Multi-family
Credit Risk Grades
Gross loans	1,553,437	794,156
Real estate loans | Pass | One-to-four family
Credit Risk Grades
Gross loans	368,270	268,776
Real estate loans | Pass | Construction
Credit Risk Grades
Gross loans	504,708	282,294
Real estate loans | Pass | Farmland
Credit Risk Grades
Gross loans	138,356	144,234
Real estate loans | Pass | Land
Credit Risk Grades
Gross loans	49,292	30,979
Real estate loans | Special Mention | Commercial non-owner occupied
Credit Risk Grades
Gross loans	163	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	1,066	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	8,102	1,070
Real estate loans | Substandard | Multi-family
Credit Risk Grades
Gross loans	1,255	228
Real estate loans | Substandard | One-to-four family
Credit Risk Grades
Gross loans	7,281	1,964
Real estate loans | Substandard | Construction
Credit Risk Grades
Gross loans	0	0
Real estate loans | Substandard | Farmland
Credit Risk Grades
Gross loans	123	1,115
Real estate loans | Substandard | Land
Credit Risk Grades
Gross loans	700	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	114,736	92,931
Consumer Loans | Pass | Consumer loans
Credit Risk Grades
Gross loans	114,598	92,794
Consumer Loans | Special Mention | Consumer loans
Credit Risk Grades
Gross loans	5	0
Consumer Loans | Substandard | Consumer loans
Credit Risk Grades
Gross loans	133	137
Consumer Loans | Doubtful | Consumer loans
Credit Risk Grades
Gross loans	$ 0	$ 0