Loans and allowance for credit losses - Loans Outstanding by Major Lending Classification (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Gross loans	$ 4,827,023		$ 4,409,642	$ 4,409,642
Less: Allowance for credit losses	(113,129)	$ (89,141)	$ (31,139)	(31,139)	$ (30,138)	$ (29,814)	$ (28,932)
Net loans	4,713,894			4,378,503
Loans processed	314,678
Paycheck Protection Program
Financing Receivable, Past Due [Line Items]
Less: Allowance for credit losses	(51)
Loans processed	314,678
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Gross loans	1,289,646			1,034,036
Less: Allowance for credit losses	(8,878)	(10,881)		(4,805)	(4,923)	(5,514)	(5,348)
Construction
Financing Receivable, Past Due [Line Items]
Gross loans	553,619			551,101
Less: Allowance for credit losses	(35,599)	(22,842)		(10,194)	(9,655)	(9,758)	(9,729)
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Gross loans	741,936			710,454
Less: Allowance for credit losses	(12,463)	(13,006)		(3,112)	(3,288)	(3,295)	(3,428)
Residential real estate: | Residential line of credit
Financing Receivable, Past Due [Line Items]
Gross loans	236,974			221,530
Less: Allowance for credit losses	(6,811)	(6,213)		(752)	(755)	(731)	(811)
Residential real estate: | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Gross loans	115,149			69,429
Less: Allowance for credit losses	(4,499)	(2,328)		(544)	(617)	(539)	(566)
Commercial real estate: | Owner occupied
Financing Receivable, Past Due [Line Items]
Gross loans	683,245			630,270
Less: Allowance for credit losses	(7,420)	(9,047)		(4,109)	(3,512)	(3,098)	(3,132)
Commercial real estate: | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Gross loans	923,192			920,744
Less: Allowance for credit losses	(30,444)	(18,005)		(4,621)	(4,478)	(4,583)	(4,149)
Consumer and other
Financing Receivable, Past Due [Line Items]
Gross loans	283,262			272,078
Less: Allowance for credit losses	$ (7,015)	$ (6,819)		$ (3,002)	$ (2,910)	$ (2,296)	$ (1,769)