Fair value measurements - Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
ARS purchase commitments
Liabilities
Liabilities Beginning Balance		$ 66		$ 195
Total Realized and Unrealized Losses		0		(1)
Purchases and Issuances		0		0
Sales and Settlements		(66)		(196)
Transfers In (Out)		0		0
Liabilities Ending Balance		0		0
Auction rate securities
Assets
Assets Beginning Balance	$ 31,977	31,422	$ 31,804	30,701
Total Realized and Unrealized Losses	(29)	(68)	(56)	(197)
Purchases and Issuances	1,175	450	1,375	2,325
Sales and Settlements	0	0	0	(1,025)
Transfers In (Out)	0	0	0	0
Assets Ending Balance	$ 33,123	$ 31,804	$ 33,123	$ 31,804