Income Taxes - Reconciliation of unrecognized tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of unrecognized tax benefits, excluding penalties and interest
Balance at the beginning of the period	$ 1,618	$ 2,070	$ 6,843
Increases during the year:
Gross increases - tax positions in prior period	57	345	712
Gross increases - current-period tax positions	22	44	331
Decreases during the year:
Gross decreases - tax positions in prior period	(92)	(135)	(4,219)
Decreases due to settlements with taxing authorities		(348)	(1,385)
Decreases due to lapse of statute of limitations	(50)	(358)	(212)
Balance at the end of the period	$ 1,555	$ 1,618	$ 2,070