Loans Held for Investment - Delinquencies (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Other information concerning the credit quality
Gross loans	$ 6,273,775	$ 6,222,053
Non-Accruing	8,149	3,284
Current
Other information concerning the credit quality
Current	6,261,021	6,211,994
30-59
Other information concerning the credit quality
Days Past Due	6,605	5,964
60-89
Other information concerning the credit quality
Days Past Due	1,084	1,056
90
Other information concerning the credit quality
Days Past Due	5,065	3,039
Business loans | Commercial and industrial
Other information concerning the credit quality
Gross loans	1,062,385	1,086,659
Non-Accruing	1,067	1,160
Business loans | Franchise
Other information concerning the credit quality
Gross loans	692,846	660,414
Non-Accruing	0	0
Business loans | Commercial owner occupied
Other information concerning the credit quality
Gross loans	1,284,528	1,295,215
Non-Accruing	3,475	97
Business loans | SBA
Other information concerning the credit quality
Gross loans	195,990	202,938
Non-Accruing	1,218	1,201
Business loans | Agribusiness
Other information concerning the credit quality
Gross loans	149,256	116,066
Non-Accruing	0	0
Business loans | Current | Commercial and industrial
Other information concerning the credit quality
Current	1,056,886	1,085,770
Business loans | Current | Franchise
Other information concerning the credit quality
Current	692,846	660,414
Business loans | Current | Commercial owner occupied
Other information concerning the credit quality
Current	1,280,900	1,291,255
Business loans | Current | SBA
Other information concerning the credit quality
Current	194,064	200,821
Business loans | Current | Agribusiness
Other information concerning the credit quality
Current	149,256	116,066
Business loans | 30-59 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	5,241	84
Business loans | 30-59 | Franchise
Other information concerning the credit quality
Days Past Due	0	0
Business loans | 30-59 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	153	3,474
Business loans | 30-59 | SBA
Other information concerning the credit quality
Days Past Due	785	177
Business loans | 30-59 | Agribusiness
Other information concerning the credit quality
Days Past Due	0	0
Business loans | 60-89 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	93	570
Business loans | 60-89 | Franchise
Other information concerning the credit quality
Days Past Due	0	0
Business loans | 60-89 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	0	486
Business loans | 60-89 | SBA
Other information concerning the credit quality
Days Past Due	991	0
Business loans | 60-89 | Agribusiness
Other information concerning the credit quality
Days Past Due	0	0
Business loans | 90 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	165	235
Business loans | 90 | Franchise
Other information concerning the credit quality
Days Past Due	0	0
Business loans | 90 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	3,475	0
Business loans | 90 | SBA
Other information concerning the credit quality
Days Past Due	150	1,940
Business loans | 90 | Agribusiness
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | Commercial non-owner occupied
Other information concerning the credit quality
Gross loans	1,227,693	1,243,115
Non-Accruing	0	0
Real estate loans | Multi-family
Other information concerning the credit quality
Gross loans	817,963	794,384
Non-Accruing	1,231	0
Real estate loans | One-to-four family
Other information concerning the credit quality
Gross loans	266,324	270,894
Non-Accruing	1,134	817
Real estate loans | Construction
Other information concerning the credit quality
Gross loans	319,610	282,811
Non-Accruing	0	0
Real estate loans | Farmland
Other information concerning the credit quality
Gross loans	136,522	145,393
Non-Accruing	0	0
Real estate loans | Land
Other information concerning the credit quality
Gross loans	34,452	31,233
Non-Accruing	8	9
Real estate loans | Current | Commercial non-owner occupied
Other information concerning the credit quality
Current	1,227,693	1,243,115
Real estate loans | Current | Multi-family
Other information concerning the credit quality
Current	816,732	792,603
Real estate loans | Current | One-to-four family
Other information concerning the credit quality
Current	265,943	269,725
Real estate loans | Current | Construction
Other information concerning the credit quality
Current	319,610	282,811
Real estate loans | Current | Farmland
Other information concerning the credit quality
Current	136,522	145,393
Real estate loans | Current | Land
Other information concerning the credit quality
Current	34,363	31,141
Real estate loans | 30-59 | Commercial non-owner occupied
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 30-59 | Multi-family
Other information concerning the credit quality
Days Past Due	0	1,781
Real estate loans | 30-59 | One-to-four family
Other information concerning the credit quality
Days Past Due	345	354
Real estate loans | 30-59 | Construction
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 30-59 | Farmland
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 30-59 | Land
Other information concerning the credit quality
Days Past Due	81	83
Real estate loans | 60-89 | Commercial non-owner occupied
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 60-89 | Multi-family
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 60-89 | One-to-four family
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 60-89 | Construction
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 60-89 | Farmland
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 60-89 | Land
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 90 | Commercial non-owner occupied
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 90 | Multi-family
Other information concerning the credit quality
Days Past Due	1,231	0
Real estate loans | 90 | One-to-four family
Other information concerning the credit quality
Days Past Due	36	815
Real estate loans | 90 | Construction
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 90 | Farmland
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 90 | Land
Other information concerning the credit quality
Days Past Due	8	9
Consumer Loans | Consumer loans
Other information concerning the credit quality
Gross loans	86,206	92,931
Non-Accruing	16	0
Consumer Loans | Current | Consumer loans
Other information concerning the credit quality
Current	86,206	92,880
Consumer Loans | 30-59 | Consumer loans
Other information concerning the credit quality
Days Past Due	0	11
Consumer Loans | 60-89 | Consumer loans
Other information concerning the credit quality
Days Past Due	0	0
Consumer Loans | 90 | Consumer loans
Other information concerning the credit quality
Days Past Due	$ 0	$ 40