Loans Held for Investment - Delinquencies (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Other information concerning the credit quality
Gross loans	$ 6,294,728	$ 6,222,053
Non-Accruing	6,039	3,284
Current
Other information concerning the credit quality
Current	6,287,281	6,211,994
30-59
Other information concerning the credit quality
Days Past Due	3,583	5,964
60-89
Other information concerning the credit quality
Days Past Due	1,290	1,056
90
Other information concerning the credit quality
Days Past Due	2,574	3,039
Business loans | Commercial and industrial
Other information concerning the credit quality
Gross loans	1,102,586	1,086,659
Non-Accruing	1,062	1,160
Business loans | Franchise
Other information concerning the credit quality
Gross loans	708,957	660,414
Non-Accruing	209	0
Business loans | Commercial owner occupied
Other information concerning the credit quality
Gross loans	1,310,722	1,295,215
Non-Accruing	0	97
Business loans | SBA
Other information concerning the credit quality
Gross loans	185,632	202,938
Non-Accruing	1,423	1,201
Business loans | Agribusiness
Other information concerning the credit quality
Gross loans	136,962	116,066
Non-Accruing	0	0
Business loans | Current | Commercial and industrial
Other information concerning the credit quality
Current	1,100,623	1,085,770
Business loans | Current | Franchise
Other information concerning the credit quality
Current	708,748	660,414
Business loans | Current | Commercial owner occupied
Other information concerning the credit quality
Current	1,310,722	1,291,255
Business loans | Current | SBA
Other information concerning the credit quality
Current	183,835	200,821
Business loans | Current | Agribusiness
Other information concerning the credit quality
Current	136,962	116,066
Business loans | 30-59 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	1,863	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	0	3,474
Business loans | 30-59 | SBA
Other information concerning the credit quality
Days Past Due	488	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	0	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	0	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	100	235
Business loans | 90 | Franchise
Other information concerning the credit quality
Days Past Due	209	0
Business loans | 90 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	0	0
Business loans | 90 | SBA
Other information concerning the credit quality
Days Past Due	1,309	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	1,224,690	1,243,115
Non-Accruing	1,290	0
Real estate loans | Multi-family
Other information concerning the credit quality
Gross loans	805,494	794,384
Non-Accruing	589	0
Real estate loans | One-to-four family
Other information concerning the credit quality
Gross loans	249,495	270,894
Non-Accruing	1,445	817
Real estate loans | Construction
Other information concerning the credit quality
Gross loans	321,423	282,811
Non-Accruing	0	0
Real estate loans | Farmland
Other information concerning the credit quality
Gross loans	136,548	145,393
Non-Accruing	0	0
Real estate loans | Land
Other information concerning the credit quality
Gross loans	30,246	31,233
Non-Accruing	6	9
Real estate loans | Current | Commercial non-owner occupied
Other information concerning the credit quality
Current	1,222,694	1,243,115
Real estate loans | Current | Multi-family
Other information concerning the credit quality
Current	804,905	792,603
Real estate loans | Current | One-to-four family
Other information concerning the credit quality
Current	248,608	269,725
Real estate loans | Current | Construction
Other information concerning the credit quality
Current	321,423	282,811
Real estate loans | Current | Farmland
Other information concerning the credit quality
Current	136,548	145,393
Real estate loans | Current | Land
Other information concerning the credit quality
Current	30,240	31,141
Real estate loans | 30-59 | Commercial non-owner occupied
Other information concerning the credit quality
Days Past Due	706	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	526	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	0	83
Real estate loans | 60-89 | Commercial non-owner occupied
Other information concerning the credit quality
Days Past Due	1,290	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	589	0
Real estate loans | 90 | One-to-four family
Other information concerning the credit quality
Days Past Due	361	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	6	9
Consumer Loans | Consumer loans
Other information concerning the credit quality
Gross loans	81,973	92,931
Non-Accruing	15	0
Consumer Loans | Current | Consumer loans
Other information concerning the credit quality
Current	81,973	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