Basic of presentation - Balance Sheet Effect (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
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	$ 805,216	$ 782,330		$ 762,329	$ 718,759	$ 694,577	$ 671,857
ASSETS
Loans	4,827,023		$ 4,409,642	4,409,642
Allowance for credit losses	(113,129)	$ (89,141)	(31,139)	(31,139)	(30,138)	$ (29,814)	(28,932)
Total assets	7,255,536			6,124,921	$ 5,940,402
Liabilities and shareholders' equity
Allowance for credit losses on unfunded commitments			0
Net deferred tax liability	80		20,490	20,490
Retained earnings	286,296		293,524	293,524
Total liabilities and shareholders' equity	$ 7,255,536			6,124,921
Period of adoption, adjusted balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				737,311			670,548
ASSETS
Loans			4,410,200
Allowance for credit losses			(62,585)
Liabilities and shareholders' equity
Allowance for credit losses on unfunded commitments			2,947
Net deferred tax liability			11,673
Retained earnings			268,506
Period of adoption, adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				$ (25,018)			$ (1,309)
ASSETS
Loans			558
Allowance for credit losses			(31,446)
Total assets			(30,888)
Liabilities and shareholders' equity
Allowance for credit losses on unfunded commitments			2,947
Net deferred tax liability			(8,817)
Retained earnings			(25,018)
Total liabilities and shareholders' equity			$ (30,888)