Fair value measurements - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
ARS purchase commitments
Liabilities
Liabilities Beginning Balance		$ 65		$ 195
Total Realized and Unrealized Gains (Losses)		(1)		(1)
Purchases and Issuances		0		0
Sales and Settlements		0		(130)
Transfers In (Out)		0		0
Liabilities Ending Balance		66		66
Auction rate securities
Assets
Assets Beginning Balance	$ 31,804	31,470	$ 31,804	30,701
Total Realized and Unrealized Losses	(27)	2	(27)	(129)
Purchases and Issuances	200	0	200	1,875
Sales and Settlements	0	(50)	0	(1,025)
Transfers In (Out)	0	0	0	0
Assets Ending Balance	$ 31,977	$ 31,422	$ 31,977	$ 31,422