Loans and allowance for credit losses - Loans Outstanding by Major Lending Classification (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Financing Receivable, Past Due [Line Items]
Gross loans	$ 4,568,038	$ 4,409,642	$ 4,409,642
Less: Allowance for credit losses	(89,141)	$ (31,139)	(31,139)	$ (29,814)	$ (28,932)
Net loans	4,478,897		4,378,503
Commercial and industrial
Financing Receivable, Past Due [Line Items]
Gross loans	1,020,484		1,034,036
Less: Allowance for credit losses	(10,881)		(4,805)	(5,514)	(5,348)
Construction
Financing Receivable, Past Due [Line Items]
Gross loans	599,479		551,101
Less: Allowance for credit losses	(22,842)		(10,194)	(9,758)	(9,729)
Residential real estate: | 1-to-4 family mortgage
Financing Receivable, Past Due [Line Items]
Gross loans	743,336		710,454
Less: Allowance for credit losses	(13,006)		(3,112)	(3,295)	(3,428)
Residential real estate: | Residential line of credit
Financing Receivable, Past Due [Line Items]
Gross loans	246,527		221,530
Less: Allowance for credit losses	(6,213)		(752)	(731)	(811)
Residential real estate: | Multi-family mortgage
Financing Receivable, Past Due [Line Items]
Gross loans	94,638		69,429
Less: Allowance for credit losses	(2,328)		(544)	(539)	(566)
Commercial real estate: | Owner occupied
Financing Receivable, Past Due [Line Items]
Gross loans	686,543		630,270
Less: Allowance for credit losses	(9,047)		(4,109)	(3,098)	(3,132)
Commercial real estate: | Non-owner occupied
Financing Receivable, Past Due [Line Items]
Gross loans	910,822		920,744
Less: Allowance for credit losses	(18,005)		(4,621)	(4,583)	(4,149)
Consumer and other
Financing Receivable, Past Due [Line Items]
Gross loans	266,209		272,078
Less: Allowance for credit losses	$ (6,819)		$ (3,002)	$ (2,296)	$ (1,769)