Basic of presentation - Balance Sheet Effect (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
ASSETS
Loans	$ 4,568,038	$ 4,409,642	$ 4,409,642
Allowance for credit losses	(89,141)	(31,139)	(31,139)	$ (29,814)	$ (28,932)
Total assets	6,655,687		6,124,921	$ 5,335,156
Liabilities and shareholders' equity
Allowance for credit losses on unfunded commitments		0
Net deferred tax liability	8,195	20,490	20,490
Retained earnings	266,385	293,524	293,524
Total liabilities and shareholders' equity	$ 6,655,687		$ 6,124,921
Cumulative Effect, Period Of Adoption, Adjusted Balance
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
Cumulative Effect, Period Of Adoption, Adjustment
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)