Loans Held for Investment - Delinquencies (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Other information concerning the credit quality
Gross loans	$ 5,054,231	$ 3,245,694
Non-Accruing	515	1,141
Commercial and industrial
Other information concerning the credit quality
Gross loans	763,091	563,169
Non-Accruing	228	250
Franchise
Other information concerning the credit quality
Gross loans	626,508	459,421
Non-Accruing	0	0
Commercial owner occupied
Other information concerning the credit quality
Gross loans	805,137	454,918
Non-Accruing	83	436
SBA
Other information concerning the credit quality
Gross loans	119,869	96,705
Non-Accruing	90	316
Agriculture
Other information concerning the credit quality
Gross loans	86,466
Non-Accruing	0
Commercial non-owner occupied
Other information concerning the credit quality
Gross loans	1,098,995	586,975
Non-Accruing	0	0
Multi-family
Other information concerning the credit quality
Gross loans	797,370	690,955
Non-Accruing	0	0
One-to-four family
Other information concerning the credit quality
Gross loans	277,933	100,451
Non-Accruing	103	124
Construction
Other information concerning the credit quality
Gross loans	301,334	269,159
Non-Accruing	0	0
Farmland
Other information concerning the credit quality
Gross loans	140,581
Non-Accruing	0
Land
Other information concerning the credit quality
Gross loans	30,719	19,829
Non-Accruing	11	15
Other loans
Other information concerning the credit quality
Gross loans	6,228	4,112
Non-Accruing	0	0
Current
Other information concerning the credit quality
Current	5,050,623	3,244,862
Current | Commercial and industrial
Other information concerning the credit quality
Current	762,588	562,805
Current | Franchise
Other information concerning the credit quality
Current	626,508	459,421
Current | Commercial owner occupied
Other information concerning the credit quality
Current	804,094	454,918
Current | SBA
Other information concerning the credit quality
Current	118,711	96,389
Current | Agriculture
Other information concerning the credit quality
Current	86,466
Current | Commercial non-owner occupied
Other information concerning the credit quality
Current	1,098,995	586,975
Current | Multi-family
Other information concerning the credit quality
Current	797,370	690,955
Current | One-to-four family
Other information concerning the credit quality
Current	277,107	100,314
Current | Construction
Other information concerning the credit quality
Current	301,334	269,159
Current | Farmland
Other information concerning the credit quality
Current	140,581
Current | Land
Other information concerning the credit quality
Current	30,709	19,814
Current | Other loans
Other information concerning the credit quality
Current	6,160	4,112
30-59
Other information concerning the credit quality
Days Past Due	556	122
30-59 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	57	104
30-59 | Franchise
Other information concerning the credit quality
Days Past Due	0	0
30-59 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	0	0
30-59 | SBA
Other information concerning the credit quality
Days Past Due	15	0
30-59 | Agriculture
Other information concerning the credit quality
Days Past Due	0
30-59 | Commercial non-owner occupied
Other information concerning the credit quality
Days Past Due	0	0
30-59 | Multi-family
Other information concerning the credit quality
Days Past Due	0	0
30-59 | One-to-four family
Other information concerning the credit quality
Days Past Due	416	18
30-59 | Construction
Other information concerning the credit quality
Days Past Due	0	0
30-59 | Farmland
Other information concerning the credit quality
Days Past Due	0
30-59 | Land
Other information concerning the credit quality
Days Past Due	0	0
30-59 | Other loans
Other information concerning the credit quality
Days Past Due	68	0
60-89
Other information concerning the credit quality
Days Past Due	1,423	71
60-89 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	119	0
60-89 | Franchise
Other information concerning the credit quality
Days Past Due	0	0
60-89 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	0	0
60-89 | SBA
Other information concerning the credit quality
Days Past Due	994	0
60-89 | Agriculture
Other information concerning the credit quality
Days Past Due	0
60-89 | Commercial non-owner occupied
Other information concerning the credit quality
Days Past Due	0	0
60-89 | Multi-family
Other information concerning the credit quality
Days Past Due	0	0
60-89 | One-to-four family
Other information concerning the credit quality
Days Past Due	310	71
60-89 | Construction
Other information concerning the credit quality
Days Past Due	0	0
60-89 | Farmland
Other information concerning the credit quality
Days Past Due	0
60-89 | Land
Other information concerning the credit quality
Days Past Due	0	0
60-89 | Other loans
Other information concerning the credit quality
Days Past Due	0	0
90
Other information concerning the credit quality
Days Past Due	1,629	639
90 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	327	260
90 | Franchise
Other information concerning the credit quality
Days Past Due	0	0
90 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	1,043	0
90 | SBA
Other information concerning the credit quality
Days Past Due	149	316
90 | Agriculture
Other information concerning the credit quality
Days Past Due	0
90 | Commercial non-owner occupied
Other information concerning the credit quality
Days Past Due	0	0
90 | Multi-family
Other information concerning the credit quality
Days Past Due	0	0
90 | One-to-four family
Other information concerning the credit quality
Days Past Due	100	48
90 | Construction
Other information concerning the credit quality
Days Past Due	0	0
90 | Farmland
Other information concerning the credit quality
Days Past Due	0
90 | Land
Other information concerning the credit quality
Days Past Due	10	15
90 | Other loans
Other information concerning the credit quality
Days Past Due	$ 0	$ 0