Loans - Internal risk grading system (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) area grade	Dec. 31, 2016 USD ($)
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	$ 6,222,053	$ 3,245,694
Pass
Credit Risk Grades
Gross loans	6,160,136	3,216,683
Special Mention
Credit Risk Grades
Gross loans	13,612	15,970
Substandard
Credit Risk Grades
Gross loans	48,305	12,791
Doubtful
Credit Risk Grades
Gross loans	0	250
Business Loans | Commercial and industrial
Credit Risk Grades
Gross loans	1,086,659	563,169
Business Loans | Franchise
Credit Risk Grades
Gross loans	660,415	459,421
Business Loans | Commercial owner occupied
Credit Risk Grades
Gross loans	1,295,214	454,918
Business Loans | SBA
Credit Risk Grades
Gross loans	202,938	96,705
Business Loans | Agribusiness
Credit Risk Grades
Gross loans	116,066
Business Loans | Pass | Commercial and industrial
Credit Risk Grades
Gross loans	1,063,452	550,919
Business Loans | Pass | Franchise
Credit Risk Grades
Gross loans	660,415	459,421
Business Loans | Pass | Commercial owner occupied
Credit Risk Grades
Gross loans	1,273,380	450,416
Business Loans | Pass | SBA
Credit Risk Grades
Gross loans	199,468	96,190
Business Loans | Pass | Agribusiness
Credit Risk Grades
Gross loans	108,143
Business Loans | Special Mention | Commercial and industrial
Credit Risk Grades
Gross loans	8,163	8,216
Business Loans | Special Mention | Franchise
Credit Risk Grades
Gross loans	0	0
Business Loans | Special Mention | Commercial owner occupied
Credit Risk Grades
Gross loans	654	281
Business Loans | Special Mention | SBA
Credit Risk Grades
Gross loans	1	53
Business Loans | Special Mention | Agribusiness
Credit Risk Grades
Gross loans	4,079
Business Loans | Substandard | Commercial and industrial
Credit Risk Grades
Gross loans	15,044	3,784
Business Loans | Substandard | Franchise
Credit Risk Grades
Gross loans	0	0
Business Loans | Substandard | Commercial owner occupied
Credit Risk Grades
Gross loans	21,180	4,221
Business Loans | Substandard | SBA
Credit Risk Grades
Gross loans	3,469	462
Business Loans | Substandard | Agribusiness
Credit Risk Grades
Gross loans	3,844
Business Loans | Doubtful | Commercial and industrial
Credit Risk Grades
Gross loans	0	250
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	1,243,115	586,975
Real Estate Loans | Multi-family
Credit Risk Grades
Gross loans	794,384	690,955
Real Estate Loans | One-to-four family
Credit Risk Grades
Gross loans	270,894	100,451
Real Estate Loans | Construction
Credit Risk Grades
Gross loans	282,811	269,159
Real Estate Loans | Farmland
Credit Risk Grades
Gross loans	145,393
Real Estate Loans | Land
Credit Risk Grades
Gross loans	31,233	19,829
Real Estate Loans | Pass | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,242,045	585,093
Real Estate Loans | Pass | Multi-family
Credit Risk Grades
Gross loans	794,156	681,942
Real Estate Loans | Pass | One-to-four family
Credit Risk Grades
Gross loans	268,776	100,010
Real Estate Loans | Pass | Construction
Credit Risk Grades
Gross loans	282,294	269,159
Real Estate Loans | Pass | Farmland
Credit Risk Grades
Gross loans	144,234
Real Estate Loans | Pass | Land
Credit Risk Grades
Gross loans	30,979	19,814
Real Estate Loans | Special Mention | Commercial non-owner occupied
Credit Risk Grades
Gross loans	0	810
Real Estate Loans | Special Mention | Multi-family
Credit Risk Grades
Gross loans	0	6,610
Real Estate Loans | Special Mention | One-to-four family
Credit Risk Grades
Gross loans	154	0
Real Estate Loans | Special Mention | Construction
Credit Risk Grades
Gross loans	517	0
Real Estate Loans | Special Mention | Farmland
Credit Risk Grades
Gross loans	44
Real Estate Loans | Special Mention | Land
Credit Risk Grades
Gross loans	0	0
Real Estate Loans | Substandard | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,070	1,072
Real Estate Loans | Substandard | Multi-family
Credit Risk Grades
Gross loans	228	2,403
Real Estate Loans | Substandard | One-to-four family
Credit Risk Grades
Gross loans	1,964	441
Real Estate Loans | Substandard | Construction
Credit Risk Grades
Gross loans	0	0
Real Estate Loans | Substandard | Farmland
Credit Risk Grades
Gross loans	1,115
Real Estate Loans | Substandard | Land
Credit Risk Grades
Gross loans	254	15
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
Real Estate Loans | Doubtful | Land
Credit Risk Grades
Gross loans	0	0
Consumer Loans | Consumer loans
Credit Risk Grades
Gross loans	92,931	4,112
Consumer Loans | Pass | Consumer loans
Credit Risk Grades
Gross loans	92,794	3,719
Consumer Loans | Special Mention | Consumer loans
Credit Risk Grades
Gross loans	0	0
Consumer Loans | Substandard | Consumer loans
Credit Risk Grades
Gross loans	137	393
Consumer Loans | Doubtful | Consumer loans
Credit Risk Grades
Gross loans	$ 0	$ 0