Loans Held for Investment - Internal Risk Grading System (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Credit Risk Grades
Gross loans	$ 5,054,231	$ 3,245,694
Commercial and industrial
Credit Risk Grades
Gross loans	763,091	563,169
Franchise
Credit Risk Grades
Gross loans	626,508	459,421
Commercial owner occupied
Credit Risk Grades
Gross loans	805,137	454,918
SBA
Credit Risk Grades
Gross loans	119,869	96,705
Agriculture
Credit Risk Grades
Gross loans	86,466
Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,098,995	586,975
Multi-family
Credit Risk Grades
Gross loans	797,370	690,955
One-to-four family
Credit Risk Grades
Gross loans	277,933	100,451
Construction and land
Credit Risk Grades
Gross loans	332,053	288,988
Farmland
Credit Risk Grades
Gross loans	140,581
Other loans
Credit Risk Grades
Gross loans	6,228	4,112
Pass
Credit Risk Grades
Gross loans	4,988,292	3,216,683
Pass | Commercial and industrial
Credit Risk Grades
Gross loans	740,405	550,919
Pass | Franchise
Credit Risk Grades
Gross loans	626,508	459,421
Pass | Commercial owner occupied
Credit Risk Grades
Gross loans	784,962	450,416
Pass | SBA
Credit Risk Grades
Gross loans	117,909	96,190
Pass | Agriculture
Credit Risk Grades
Gross loans	80,344
Pass | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,095,919	585,093
Pass | Multi-family
Credit Risk Grades
Gross loans	796,603	681,942
Pass | One-to-four family
Credit Risk Grades
Gross loans	276,511	100,010
Pass | Construction and land
Credit Risk Grades
Gross loans	324,773	288,973
Pass | Farmland
Credit Risk Grades
Gross loans	138,409
Pass | Other loans
Credit Risk Grades
Gross loans	5,949	3,719
Special Mention
Credit Risk Grades
Gross loans	23,255	15,970
Special Mention | Commercial and industrial
Credit Risk Grades
Gross loans	14,694	8,216
Special Mention | Franchise
Credit Risk Grades
Gross loans	0	0
Special Mention | Commercial owner occupied
Credit Risk Grades
Gross loans	983	281
Special Mention | SBA
Credit Risk Grades
Gross loans	222	53
Special Mention | Agriculture
Credit Risk Grades
Gross loans	4,433
Special Mention | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,985	810
Special Mention | Multi-family
Credit Risk Grades
Gross loans	0	6,610
Special Mention | One-to-four family
Credit Risk Grades
Gross loans	578	0
Special Mention | Construction and land
Credit Risk Grades
Gross loans	299	0
Special Mention | Farmland
Credit Risk Grades
Gross loans	61
Special Mention | Other loans
Credit Risk Grades
Gross loans	0	0
Substandard
Credit Risk Grades
Gross loans	42,684	12,791
Substandard | Commercial and industrial
Credit Risk Grades
Gross loans	7,992	3,784
Substandard | Franchise
Credit Risk Grades
Gross loans	0	0
Substandard | Commercial owner occupied
Credit Risk Grades
Gross loans	19,192	4,221
Substandard | SBA
Credit Risk Grades
Gross loans	1,738	462
Substandard | Agriculture
Credit Risk Grades
Gross loans	1,689
Substandard | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,091	1,072
Substandard | Multi-family
Credit Risk Grades
Gross loans	767	2,403
Substandard | One-to-four family
Credit Risk Grades
Gross loans	844	441
Substandard | Construction and land
Credit Risk Grades
Gross loans	6,981	15
Substandard | Farmland
Credit Risk Grades
Gross loans	2,111
Substandard | Other loans
Credit Risk Grades
Gross loans	279	393
Doubtful
Credit Risk Grades
Gross loans	0	250
Doubtful | Commercial and industrial
Credit Risk Grades
Gross loans	0	250
Doubtful | Franchise
Credit Risk Grades
Gross loans	0	0
Doubtful | Commercial owner occupied
Credit Risk Grades
Gross loans	0	0
Doubtful | SBA
Credit Risk Grades
Gross loans	0	0
Doubtful | Agriculture
Credit Risk Grades
Gross loans	0
Doubtful | Commercial non-owner occupied
Credit Risk Grades
Gross loans	0	0
Doubtful | Multi-family
Credit Risk Grades
Gross loans	0	0
Doubtful | One-to-four family
Credit Risk Grades
Gross loans	0	0
Doubtful | Construction and land
Credit Risk Grades
Gross loans	0	0
Doubtful | Farmland
Credit Risk Grades
Gross loans	0
Doubtful | Other loans
Credit Risk Grades
Gross loans	$ 0	$ 0