Loans Held for Investment - Delinquencies (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Other information concerning the credit quality
Gross loans	$ 8,790,435	$ 8,852,005
Non-Accruing	7,637	4,857
Current
Other information concerning the credit quality
Current	8,776,957	8,839,152
30-59
Other information concerning the credit quality
Days Past Due	3,416	7,046
60-89
Other information concerning the credit quality
Days Past Due	801	1,242
90
Other information concerning the credit quality
Days Past Due	9,261	4,565
Business loans | Commercial and industrial
Other information concerning the credit quality
Gross loans	1,300,083	1,364,461
Non-Accruing	2,935	931
Business loans | Franchise
Other information concerning the credit quality
Gross loans	860,299	765,416
Non-Accruing	697	190
Business loans | Commercial owner occupied
Other information concerning the credit quality
Gross loans	1,667,912	1,679,122
Non-Accruing	564	599
Business loans | SBA
Other information concerning the credit quality
Gross loans	187,530	198,201
Non-Accruing	2,338	2,739
Business loans | Agribusiness
Other information concerning the credit quality
Gross loans	126,857	138,519
Non-Accruing	0	0
Business loans | Current | Commercial and industrial
Other information concerning the credit quality
Current	1,293,042	1,362,017
Business loans | Current | Franchise
Other information concerning the credit quality
Current	859,289	759,546
Business loans | Current | Commercial owner occupied
Other information concerning the credit quality
Current	1,666,135	1,677,967
Business loans | Current | SBA
Other information concerning the credit quality
Current	184,613	195,051
Business loans | Current | Agribusiness
Other information concerning the credit quality
Current	126,857	138,519
Business loans | 30-59 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	1,841	309
Business loans | 30-59 | Franchise
Other information concerning the credit quality
Days Past Due	0	5,680
Business loans | 30-59 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	1,054	343
Business loans | 30-59 | SBA
Other information concerning the credit quality
Days Past Due	511	524
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	416	1,204
Business loans | 60-89 | Franchise
Other information concerning the credit quality
Days Past Due	313	0
Business loans | 60-89 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	0	0
Business loans | 60-89 | SBA
Other information concerning the credit quality
Days Past Due	72	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	4,784	931
Business loans | 90 | Franchise
Other information concerning the credit quality
Days Past Due	697	190
Business loans | 90 | Commercial owner occupied
Other information concerning the credit quality
Days Past Due	723	812
Business loans | 90 | SBA
Other information concerning the credit quality
Days Past Due	2,334	2,626
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	2,122,674	2,004,536
Non-Accruing	243	0
Real estate loans | Multi-family
Other information concerning the credit quality
Gross loans	1,520,135	1,535,289
Non-Accruing	0	0
Real estate loans | One-to-four family
Other information concerning the credit quality
Gross loans	248,392	356,264
Non-Accruing	380	398
Real estate loans | Construction
Other information concerning the credit quality
Gross loans	505,401	523,643
Non-Accruing	0	0
Real estate loans | Farmland
Other information concerning the credit quality
Gross loans	169,724	150,502
Non-Accruing	0	0
Real estate loans | Land
Other information concerning the credit quality
Gross loans	40,748	46,628
Non-Accruing	480	0
Real estate loans | Current | Commercial non-owner occupied
Other information concerning the credit quality
Current	2,122,431	2,004,536
Real estate loans | Current | Multi-family
Other information concerning the credit quality
Current	1,520,135	1,535,275
Real estate loans | Current | One-to-four family
Other information concerning the credit quality
Current	248,392	356,219
Real estate loans | Current | Construction
Other information concerning the credit quality
Current	505,401	523,643
Real estate loans | Current | Farmland
Other information concerning the credit quality
Current	169,724	150,502
Real estate loans | Current | Land
Other information concerning the credit quality
Current	40,268	46,628
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	14
Real estate loans | 30-59 | One-to-four family
Other information concerning the credit quality
Days Past Due	0	30
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	0
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	9
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	243	0
Real estate loans | 90 | Multi-family
Other information concerning the credit quality
Days Past Due	0	0
Real estate loans | 90 | One-to-four family
Other information concerning the credit quality
Days Past Due	0	6
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	480	0
Consumer Loans | Consumer loans
Other information concerning the credit quality
Gross loans	40,680	89,424
Non-Accruing	0	0
Consumer Loans | Current | Consumer loans
Other information concerning the credit quality
Current	40,670	89,249
Consumer Loans | 30-59 | Consumer loans
Other information concerning the credit quality
Days Past Due	10	146
Consumer Loans | 60-89 | Consumer loans
Other information concerning the credit quality
Days Past Due	0	29
Consumer Loans | 90 | Consumer loans
Other information concerning the credit quality
Days Past Due	$ 0	$ 0