Loans - Internal risk grading system (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) area grade	Dec. 31, 2017 USD ($)
Receivables [Abstract]
Number of areas where the entity's credit quality is maintained and credit risk managed | area	2
Number of Pass scale grades | grade	6
Credit Risk Grades
Gross loans	$ 8,852,005	$ 6,222,053
Warehouse Facilities
Credit Risk Grades
Gross loans		116,066
Pass
Credit Risk Grades
Gross loans	8,764,593	6,160,136
Pass | Warehouse Facilities
Credit Risk Grades
Gross loans		108,143
Special Mention
Credit Risk Grades
Gross loans	25,956	13,612
Special Mention | Warehouse Facilities
Credit Risk Grades
Gross loans		4,079
Substandard
Credit Risk Grades
Gross loans	61,456	48,305
Substandard | Warehouse Facilities
Credit Risk Grades
Gross loans		3,844
Doubtful
Credit Risk Grades
Gross loans	0	0
Doubtful | Warehouse Facilities
Credit Risk Grades
Gross loans		0
Business Loans | Commercial and industrial
Credit Risk Grades
Gross loans	1,364,461	1,086,659
Business Loans | Franchise
Credit Risk Grades
Gross loans	765,416	660,415
Business Loans | Commercial owner occupied
Credit Risk Grades
Gross loans	1,679,122	1,295,214
Business Loans | SBA
Credit Risk Grades
Gross loans	198,201	202,938
Business Loans | Warehouse Facilities
Credit Risk Grades
Gross loans		116,066
Business Loans | Agribusiness
Credit Risk Grades
Gross loans	138,519
Business Loans | Pass | Commercial and industrial
Credit Risk Grades
Gross loans	1,340,322	1,063,452
Business Loans | Pass | Franchise
Credit Risk Grades
Gross loans	760,795	660,415
Business Loans | Pass | Commercial owner occupied
Credit Risk Grades
Gross loans	1,660,994	1,273,380
Business Loans | Pass | SBA
Credit Risk Grades
Gross loans	189,006	199,468
Business Loans | Pass | Agribusiness
Credit Risk Grades
Gross loans	125,355
Business Loans | Special Mention | Commercial and industrial
Credit Risk Grades
Gross loans	12,005	8,163
Business Loans | Special Mention | Franchise
Credit Risk Grades
Gross loans	4,431	0
Business Loans | Special Mention | Commercial owner occupied
Credit Risk Grades
Gross loans	1,580	654
Business Loans | Special Mention | SBA
Credit Risk Grades
Gross loans	2,289	1
Business Loans | Special Mention | Agribusiness
Credit Risk Grades
Gross loans	0
Business Loans | Substandard | Commercial and industrial
Credit Risk Grades
Gross loans	12,134	15,044
Business Loans | Substandard | Franchise
Credit Risk Grades
Gross loans	190	0
Business Loans | Substandard | Commercial owner occupied
Credit Risk Grades
Gross loans	16,548	21,180
Business Loans | Substandard | SBA
Credit Risk Grades
Gross loans	6,906	3,469
Business Loans | Substandard | Agribusiness
Credit Risk Grades
Gross loans	13,164
Business Loans | Doubtful | Commercial and industrial
Credit Risk Grades
Gross loans	0	0
Business Loans | Doubtful | Franchise
Credit Risk Grades
Gross loans	0	0
Business Loans | Doubtful | Commercial owner occupied
Credit Risk Grades
Gross loans	0	0
Business Loans | Doubtful | SBA
Credit Risk Grades
Gross loans	0	0
Business Loans | Doubtful | Agribusiness
Credit Risk Grades
Gross loans	0
Real Estate Loans | Commercial non-owner occupied
Credit Risk Grades
Gross loans	2,004,536	1,243,115
Real Estate Loans | Multi-family
Credit Risk Grades
Gross loans	1,535,289	794,384
Real Estate Loans | One-to-four family
Credit Risk Grades
Gross loans	356,264	270,894
Real Estate Loans | Construction
Credit Risk Grades
Gross loans	523,643	282,811
Real Estate Loans | Farmland
Credit Risk Grades
Gross loans	150,502	145,393
Real Estate Loans | Land
Credit Risk Grades
Gross loans	46,628	31,233
Real Estate Loans | Pass | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,998,118	1,242,045
Real Estate Loans | Pass | Multi-family
Credit Risk Grades
Gross loans	1,530,567	794,156
Real Estate Loans | Pass | One-to-four family
Credit Risk Grades
Gross loans	350,083	268,776
Real Estate Loans | Pass | Construction
Credit Risk Grades
Gross loans	523,643	282,294
Real Estate Loans | Pass | Farmland
Credit Risk Grades
Gross loans	150,381	144,234
Real Estate Loans | Pass | Land
Credit Risk Grades
Gross loans	46,008	30,979
Real Estate Loans | Special Mention | Commercial non-owner occupied
Credit Risk Grades
Gross loans	731	0
Real Estate Loans | Special Mention | Multi-family
Credit Risk Grades
Gross loans	4,060	0
Real Estate Loans | Special Mention | One-to-four family
Credit Risk Grades
Gross loans	728	154
Real Estate Loans | Special Mention | Construction
Credit Risk Grades
Gross loans	0	517
Real Estate Loans | Special Mention | Farmland
Credit Risk Grades
Gross loans	0	44
Real Estate Loans | Special Mention | Land
Credit Risk Grades
Gross loans	132	0
Real Estate Loans | Substandard | Commercial non-owner occupied
Credit Risk Grades
Gross loans	5,687	1,070
Real Estate Loans | Substandard | Multi-family
Credit Risk Grades
Gross loans	662	228
Real Estate Loans | Substandard | One-to-four family
Credit Risk Grades
Gross loans	5,453	1,964
Real Estate Loans | Substandard | Construction
Credit Risk Grades
Gross loans	0	0
Real Estate Loans | Substandard | Farmland
Credit Risk Grades
Gross loans	121	1,115
Real Estate Loans | Substandard | Land
Credit Risk Grades
Gross loans	488	254
Real Estate Loans | Doubtful | Commercial non-owner occupied
Credit Risk Grades
Gross loans	0	0
Real Estate Loans | Doubtful | Multi-family
Credit Risk Grades
Gross loans	0	0
Real Estate Loans | Doubtful | One-to-four family
Credit Risk Grades
Gross loans	0	0
Real Estate Loans | Doubtful | Construction
Credit Risk Grades
Gross loans	0	0
Real Estate Loans | Doubtful | Farmland
Credit Risk Grades
Gross loans	0	0
Real Estate Loans | Doubtful | Land
Credit Risk Grades
Gross loans	0	0
Consumer Loans | Consumer loans
Credit Risk Grades
Gross loans	89,424	92,931
Consumer Loans | Pass | Consumer loans
Credit Risk Grades
Gross loans	89,321	92,794
Consumer Loans | Special Mention | Consumer loans
Credit Risk Grades
Gross loans	0	0
Consumer Loans | Substandard | Consumer loans
Credit Risk Grades
Gross loans	103	137
Consumer Loans | Doubtful | Consumer loans
Credit Risk Grades
Gross loans	$ 0	$ 0