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	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 0	$ 3,604
Reclassification of equity securities without a readily determinable fair value to other assets	0	(3,604)
Balance at end of period	$ 0	$ 0