Loans - Internal risk grading system (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) area grade	Dec. 31, 2018 USD ($)
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	$ 8,733,570	$ 8,852,005
Pass
Credit Risk Grades
Gross loans	8,651,947	8,764,593
Special Mention
Credit Risk Grades
Gross loans	36,677	25,956
Substandard
Credit Risk Grades
Gross loans	44,946	61,456
Doubtful
Credit Risk Grades
Gross loans	0	0
Business Loans | Commercial and industrial
Credit Risk Grades
Gross loans	1,265,185	1,364,461
Business Loans | Franchise
Credit Risk Grades
Gross loans	916,875	765,416
Business Loans | Commercial owner occupied
Credit Risk Grades
Gross loans	1,674,092	1,679,122
Business Loans | SBA
Credit Risk Grades
Gross loans	177,487	198,201
Business Loans | Agribusiness
Credit Risk Grades
Gross loans	127,834
Business Loans | Warehouse Facilities
Credit Risk Grades
Gross loans		138,519
Business Loans | Pass | Commercial and industrial
Credit Risk Grades
Gross loans	1,236,073	1,340,322
Business Loans | Pass | Franchise
Credit Risk Grades
Gross loans	898,191	760,795
Business Loans | Pass | Commercial owner occupied
Credit Risk Grades
Gross loans	1,659,391	1,660,994
Business Loans | Pass | SBA
Credit Risk Grades
Gross loans	166,011	189,006
Business Loans | Pass | Agribusiness
Credit Risk Grades
Gross loans	123,338
Business Loans | Pass | Warehouse Facilities
Credit Risk Grades
Gross loans		125,355
Business Loans | Special Mention | Commercial and industrial
Credit Risk Grades
Gross loans	13,226	12,005
Business Loans | Special Mention | Franchise
Credit Risk Grades
Gross loans	7,851	4,431
Business Loans | Special Mention | Commercial owner occupied
Credit Risk Grades
Gross loans	11,167	1,580
Business Loans | Special Mention | SBA
Credit Risk Grades
Gross loans	3,255	2,289
Business Loans | Special Mention | Agribusiness
Credit Risk Grades
Gross loans	0
Business Loans | Special Mention | Warehouse Facilities
Credit Risk Grades
Gross loans		0
Business Loans | Substandard | Commercial and industrial
Credit Risk Grades
Gross loans	15,886	12,134
Business Loans | Substandard | Franchise
Credit Risk Grades
Gross loans	10,833	190
Business Loans | Substandard | Commercial owner occupied
Credit Risk Grades
Gross loans	3,534	16,548
Business Loans | Substandard | SBA
Credit Risk Grades
Gross loans	8,221	6,906
Business Loans | Substandard | Agribusiness
Credit Risk Grades
Gross loans	4,496
Business Loans | Substandard | Warehouse Facilities
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
Business Loans | Doubtful | Warehouse Facilities
Credit Risk Grades
Gross loans		0
Real Estate Loans | Commercial non-owner occupied
Credit Risk Grades
Gross loans	2,072,374	2,004,536
Real Estate Loans | Multi-family
Credit Risk Grades
Gross loans	1,576,870	1,535,289
Real Estate Loans | One-to-four family
Credit Risk Grades
Gross loans	254,779	356,264
Real Estate Loans | Construction
Credit Risk Grades
Gross loans	410,065	523,643
Real Estate Loans | Farmland
Credit Risk Grades
Gross loans	175,997	150,502
Real Estate Loans | Land
Credit Risk Grades
Gross loans	31,090	46,628
Real Estate Loans | Pass | Commercial non-owner occupied
Credit Risk Grades
Gross loans	2,070,068	1,998,118
Real Estate Loans | Pass | Multi-family
Credit Risk Grades
Gross loans	1,576,654	1,530,567
Real Estate Loans | Pass | One-to-four family
Credit Risk Grades
Gross loans	254,218	350,083
Real Estate Loans | Pass | Construction
Credit Risk Grades
Gross loans	410,065	523,643
Real Estate Loans | Pass | Farmland
Credit Risk Grades
Gross loans	175,997	150,381
Real Estate Loans | Pass | Land
Credit Risk Grades
Gross loans	31,073	46,008
Real Estate Loans | Special Mention | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,178	731
Real Estate Loans | Special Mention | Multi-family
Credit Risk Grades
Gross loans	0	4,060
Real Estate Loans | Special Mention | One-to-four family
Credit Risk Grades
Gross loans	0	728
Real Estate Loans | Special Mention | Construction
Credit Risk Grades
Gross loans	0	0
Real Estate Loans | Special Mention | Farmland
Credit Risk Grades
Gross loans	0	0
Real Estate Loans | Special Mention | Land
Credit Risk Grades
Gross loans	0	132
Real Estate Loans | Substandard | Commercial non-owner occupied
Credit Risk Grades
Gross loans	1,128	5,687
Real Estate Loans | Substandard | Multi-family
Credit Risk Grades
Gross loans	216	662
Real Estate Loans | Substandard | One-to-four family
Credit Risk Grades
Gross loans	561	5,453
Real Estate Loans | Substandard | Construction
Credit Risk Grades
Gross loans	0	0
Real Estate Loans | Substandard | Farmland
Credit Risk Grades
Gross loans	0	121
Real Estate Loans | Substandard | Land
Credit Risk Grades
Gross loans	17	488
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	50,922	89,424
Consumer Loans | Pass | Consumer loans
Credit Risk Grades
Gross loans	50,868	89,321
Consumer Loans | Special Mention | Consumer loans
Credit Risk Grades
Gross loans	0	0
Consumer Loans | Substandard | Consumer loans
Credit Risk Grades
Gross loans	54	103
Consumer Loans | Doubtful | Consumer loans
Credit Risk Grades
Gross loans	$ 0	$ 0