Fair value of financial instruments - Reconciliation for Assets and Liabilities Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs or Level 3 Inputs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 3,604	$ 4,549
Reclassification of equity securities without a readily determinable fair value to other assets (1)	(3,604)	0
Impairment of equity securities	0	(945)
Balance at end of period	$ 0	$ 3,604