Loans Held for Investment - Internal Risk Grading System (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Credit Risk Grades
Gross loans	$ 4,864,883	$ 3,245,694	$ 2,927,554
Commercial and industrial
Credit Risk Grades
Gross loans	733,852	563,169	508,141
Franchise
Credit Risk Grades
Gross loans	565,415	459,421	403,855
Commercial owner occupied
Credit Risk Grades
Gross loans	729,476	454,918	443,060
SBA
Credit Risk Grades
Gross loans	108,224	96,705	86,076
Agriculture
Credit Risk Grades
Gross loans	98,842	0
Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,095,184	586,975	526,362
Multi-family
Credit Risk Grades
Gross loans	746,547	690,955	613,573
One-to-four family
Credit Risk Grades
Gross loans	322,048	100,451	106,538
Construction and land
Credit Risk Grades
Gross loans	321,399	288,988
Farmland
Credit Risk Grades
Gross loans	136,587	0
Other loans
Credit Risk Grades
Gross loans	7,309	4,112	$ 5,822
Pass
Credit Risk Grades
Gross loans	4,809,793	3,216,683
Pass | Commercial and industrial
Credit Risk Grades
Gross loans	715,759	550,919
Pass | Franchise
Credit Risk Grades
Gross loans	565,415	459,421
Pass | Commercial owner occupied
Credit Risk Grades
Gross loans	711,959	450,416
Pass | SBA
Credit Risk Grades
Gross loans	107,429	96,190
Pass | Agriculture
Credit Risk Grades
Gross loans	92,522
Pass | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,087,928	585,093
Pass | Multi-family
Credit Risk Grades
Gross loans	745,766	681,942
Pass | One-to-four family
Credit Risk Grades
Gross loans	321,021	100,010
Pass | Construction and land
Credit Risk Grades
Gross loans	320,610	288,973
Pass | Farmland
Credit Risk Grades
Gross loans	134,409
Pass | Other loans
Credit Risk Grades
Gross loans	6,975	3,719
Special Mention
Credit Risk Grades
Gross loans	21,683	15,970
Special Mention | Commercial and industrial
Credit Risk Grades
Gross loans	8,972	8,216
Special Mention | Franchise
Credit Risk Grades
Gross loans	0	0
Special Mention | Commercial owner occupied
Credit Risk Grades
Gross loans	2,035	281
Special Mention | SBA
Credit Risk Grades
Gross loans	10	53
Special Mention | Agriculture
Credit Risk Grades
Gross loans	4,634
Special Mention | Commercial non-owner occupied
Credit Risk Grades
Gross loans	5,784	810
Special Mention | Multi-family
Credit Risk Grades
Gross loans	0	6,610
Special Mention | One-to-four family
Credit Risk Grades
Gross loans	171	0
Special Mention | Construction and land
Credit Risk Grades
Gross loans	0	0
Special Mention | Farmland
Credit Risk Grades
Gross loans	77
Special Mention | Other loans
Credit Risk Grades
Gross loans	0	0
Substandard
Credit Risk Grades
Gross loans	33,407	12,791
Substandard | Commercial and industrial
Credit Risk Grades
Gross loans	9,121	3,784
Substandard | Franchise
Credit Risk Grades
Gross loans	0	0
Substandard | Commercial owner occupied
Credit Risk Grades
Gross loans	15,482	4,221
Substandard | SBA
Credit Risk Grades
Gross loans	785	462
Substandard | Agriculture
Credit Risk Grades
Gross loans	1,686
Substandard | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,472	1,072
Substandard | Multi-family
Credit Risk Grades
Gross loans	781	2,403
Substandard | One-to-four family
Credit Risk Grades
Gross loans	856	441
Substandard | Construction and land
Credit Risk Grades
Gross loans	789	15
Substandard | Farmland
Credit Risk Grades
Gross loans	2,101
Substandard | Other loans
Credit Risk Grades
Gross loans	334	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 | Other loans
Credit Risk Grades
Gross loans	$ 0	$ 0