Basic of presentation - Balance Sheet Effect (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	$ 1,244,998			$ 762,329
ASSETS
Loans	7,213,538		$ 4,409,642	4,409,642
Allowance for credit losses	(183,973)	$ (113,129)	(31,139)	(31,139)	$ (31,464)	$ (30,138)	$ (28,932)
Total assets	11,010,438			6,124,921	$ 6,088,895
Liabilities and shareholders' equity
Allowance for credit losses on unfunded commitments			0
Net deferred tax liability			20,490	20,490
Retained earnings	276,361		293,524	293,524
Total liabilities and shareholders' equity	$ 11,010,438			$ 6,124,921
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)
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