Loans - Days past due and Non-accruing (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Other information concerning the credit quality
Total Gross Loans	$ 6,222,053	$ 3,245,694
Non-accruing	3,284	1,141
Current
Other information concerning the credit quality
Current	6,211,994	3,244,862
30-59
Other information concerning the credit quality
Days Past Due	5,964	122
60-89
Other information concerning the credit quality
Days Past Due	1,056	71
90
Other information concerning the credit quality
Days Past Due	3,039	639
Business Loans | Commercial and industrial
Other information concerning the credit quality
Total Gross Loans	1,086,659	563,169
Non-accruing	1,160	250
Business Loans | Franchise
Other information concerning the credit quality
Total Gross Loans	660,415	459,421
Non-accruing	0	0
Business Loans | Commercial owner occupied
Other information concerning the credit quality
Total Gross Loans	1,295,214	454,918
Non-accruing	97	436
Business Loans | SBA
Other information concerning the credit quality
Total Gross Loans	202,938	96,705
Non-accruing	1,201	316
Business Loans | Agribusiness
Other information concerning the credit quality
Total Gross Loans	116,066
Non-accruing	0
Business Loans | Current | Commercial and industrial
Other information concerning the credit quality
Current	1,085,770	562,805
Business Loans | Current | Franchise
Other information concerning the credit quality
Current	660,415	459,421
Business Loans | Current | Commercial owner occupied
Other information concerning the credit quality
Current	1,291,254	454,918
Business Loans | Current | SBA
Other information concerning the credit quality
Current	200,821	96,389
Business Loans | Current | Agribusiness
Other information concerning the credit quality
Current	116,066
Business Loans | 30-59 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	84	104
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	3,474	0
Business Loans | 30-59 | SBA
Other information concerning the credit quality
Days Past Due	177	0
Business Loans | 30-59 | Agribusiness
Other information concerning the credit quality
Days Past Due	0
Business Loans | 60-89 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	570	0
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	486	0
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
Business Loans | 90 | Commercial and industrial
Other information concerning the credit quality
Days Past Due	235	260
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	0	0
Business Loans | 90 | SBA
Other information concerning the credit quality
Days Past Due	1,940	316
Business Loans | 90 | Agribusiness
Other information concerning the credit quality
Days Past Due	0
Real Estate Loans | Commercial non-owner occupied
Other information concerning the credit quality
Total Gross Loans	1,243,115	586,975
Non-accruing	0	0
Real Estate Loans | Multi-family
Other information concerning the credit quality
Total Gross Loans	794,384	690,955
Non-accruing	0	0
Real Estate Loans | One-to-four family
Other information concerning the credit quality
Total Gross Loans	270,894	100,451
Non-accruing	817	124
Real Estate Loans | Construction
Other information concerning the credit quality
Total Gross Loans	282,811	269,159
Non-accruing	0	0
Real Estate Loans | Farmland
Other information concerning the credit quality
Total Gross Loans	145,393
Non-accruing	0
Real Estate Loans | Land
Other information concerning the credit quality
Total Gross Loans	31,233	19,829
Non-accruing	9	15
Real Estate Loans | Current | Commercial non-owner occupied
Other information concerning the credit quality
Current	1,243,115	586,975
Real Estate Loans | Current | Multi-family
Other information concerning the credit quality
Current	792,603	690,955
Real Estate Loans | Current | One-to-four family
Other information concerning the credit quality
Current	269,725	100,314
Real Estate Loans | Current | Construction
Other information concerning the credit quality
Current	282,811	269,159
Real Estate Loans | Current | Farmland
Other information concerning the credit quality
Current	145,393
Real Estate Loans | Current | Land
Other information concerning the credit quality
Current	31,141	19,814
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	1,781	0
Real Estate Loans | 30-59 | One-to-four family
Other information concerning the credit quality
Days Past Due	354	18
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
Real Estate Loans | 30-59 | Land
Other information concerning the credit quality
Days Past Due	83	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	71
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
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	0	0
Real Estate Loans | 90 | One-to-four family
Other information concerning the credit quality
Days Past Due	815	48
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
Real Estate Loans | 90 | Land
Other information concerning the credit quality
Days Past Due	9	15
Consumer Loans | Consumer loans
Other information concerning the credit quality
Total Gross Loans	92,931	4,112
Non-accruing	0	0
Consumer Loans | Current | Consumer loans
Other information concerning the credit quality
Current	92,880	4,112
Consumer Loans | 30-59 | Consumer loans
Other information concerning the credit quality
Days Past Due	11	0
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	$ 40	$ 0