Loans Held for Investment - Delinquencies (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Other information concerning the credit quality
Gross loans	$ 8,820,732	$ 6,222,053
Non-Accruing	7,268	3,284
Current
Other information concerning the credit quality
Current	8,812,987	6,211,994
30-59
Other information concerning the credit quality
Days Past Due	1,977	5,964
60-89
Other information concerning the credit quality
Days Past Due	720	1,056
90
Other information concerning the credit quality
Days Past Due	5,048	3,039
Business loans | Commercial and industrial
Other information concerning the credit quality
Gross loans	1,359,841	1,086,659
Non-Accruing	1,027	1,160
Business loans | Franchise
Other information concerning the credit quality
Gross loans	735,366	660,414
Non-Accruing	209	0
Business loans | Commercial owner occupied
Other information concerning the credit quality
Gross loans	1,717,150	1,295,215
Non-Accruing	0	97
Business loans | SBA
Other information concerning the credit quality
Gross loans	203,229	202,938
Non-Accruing	2,748	1,201
Business loans | Agribusiness
Other information concerning the credit quality
Gross loans	133,241	116,066
Non-Accruing	0	0
Business loans | Current | Commercial and industrial
Other information concerning the credit quality
Current	1,358,766	1,085,770
Business loans | Current | Franchise
Other information concerning the credit quality
Current	735,157	660,414
Business loans | Current | Commercial owner occupied
Other information concerning the credit quality
Current	1,716,146	1,291,255
Business loans | Current | SBA
Other information concerning the credit quality
Current	200,596	200,821
Business loans | Current | Agribusiness
Other information concerning the credit quality
Current	133,241	116,066
Business loans | 30-59 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	334	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	793	3,474
Business loans | 30-59 | SBA
Other information concerning the credit quality
Days Past Due	4	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	636	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	105	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	211	0
Business loans | 90 | SBA
Other information concerning the credit quality
Days Past Due	2,629	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	1,932,681	1,243,115
Non-Accruing	1,290	0
Real estate loans | Multi-family
Other information concerning the credit quality
Gross loans	1,554,692	794,384
Non-Accruing	589	0
Real estate loans | One-to-four family
Other information concerning the credit quality
Gross loans	376,617	270,894
Non-Accruing	1,388	817
Real estate loans | Construction
Other information concerning the credit quality
Gross loans	504,708	282,811
Non-Accruing	0	0
Real estate loans | Farmland
Other information concerning the credit quality
Gross loans	138,479	145,393
Non-Accruing	0	0
Real estate loans | Land
Other information concerning the credit quality
Gross loans	49,992	31,233
Non-Accruing	4	9
Real estate loans | Current | Commercial non-owner occupied
Other information concerning the credit quality
Current	1,931,391	1,243,115
Real estate loans | Current | Multi-family
Other information concerning the credit quality
Current	1,554,103	792,603
Real estate loans | Current | One-to-four family
Other information concerning the credit quality
Current	375,694	269,725
Real estate loans | Current | Construction
Other information concerning the credit quality
Current	504,708	282,811
Real estate loans | Current | Farmland
Other information concerning the credit quality
Current	138,479	145,393
Real estate loans | Current | Land
Other information concerning the credit quality
Current	49,988	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	836	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	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	76	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	1,290	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	11	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	4	9
Consumer Loans | Consumer loans
Other information concerning the credit quality
Gross loans	114,736	92,931
Non-Accruing	13	0
Consumer Loans | Current | Consumer loans
Other information concerning the credit quality
Current	114,718	92,880
Consumer Loans | 30-59 | Consumer loans
Other information concerning the credit quality
Days Past Due	10	11
Consumer Loans | 60-89 | Consumer loans
Other information concerning the credit quality
Days Past Due	8	0
Consumer Loans | 90 | Consumer loans
Other information concerning the credit quality
Days Past Due	$ 0	$ 40