Summary of significant accounting policies - Recent Accounting Pronouncements Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ 101,201		$ 53,355	$ 52,119	$ 53,637
Allowance for loan commitments	4,300		1,741
Decrease to retained earnings	$ (660,398)	$ (622,042)	(622,042)
Impact of ASU No. 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		21,000	19,441
Allowance for loan commitments		2,000	$ 1,559
Decrease to retained earnings		$ 15,372