Fair Value of Financial Instruments - Schedule of assets measured on a recurring basis at fair value (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Cash equivalents
Total cash equivalents	$ 79,039	$ 113,402
Restricted cash equivalents
Total restricted cash equivalents	17,693	22,900
Marketable securities - restricted
Total marketable securities - restricted	67,149	62,625
Money market accounts
Cash equivalents
Total cash equivalents	79,039	113,402
Restricted cash equivalents
Total restricted cash equivalents	14,662	19,569
Certificates of deposits
Restricted cash equivalents
Total restricted cash equivalents	3,031	3,331
Corporate debt securities
Marketable securities - restricted
Total marketable securities - restricted	2,738	2,545
U.S. treasury and agency securities
Marketable securities - restricted
Total marketable securities - restricted	39,649	35,281
Commercial paper
Marketable securities - restricted
Total marketable securities - restricted	18,550	16,081
Asset backed securities
Marketable securities - restricted
Total marketable securities - restricted	6,212	8,718
Level 1
Cash equivalents
Total cash equivalents	79,039	113,402
Restricted cash equivalents
Total restricted cash equivalents	17,693	22,900
Marketable securities - restricted
Total marketable securities - restricted	37,693	33,295
Level 1 | Money market accounts
Cash equivalents
Total cash equivalents	79,039	113,402
Restricted cash equivalents
Total restricted cash equivalents	14,662	19,569
Level 1 | Certificates of deposits
Restricted cash equivalents
Total restricted cash equivalents	3,031	3,331
Level 1 | Corporate debt securities
Marketable securities - restricted
Total marketable securities - restricted	0	0
Level 1 | U.S. treasury and agency securities
Marketable securities - restricted
Total marketable securities - restricted	37,693	33,295
Level 1 | Commercial paper
Marketable securities - restricted
Total marketable securities - restricted	0	0
Level 1 | Asset backed securities
Marketable securities - restricted
Total marketable securities - restricted	0	0
Level 2
Cash equivalents
Total cash equivalents	0	0
Restricted cash equivalents
Total restricted cash equivalents	0	0
Marketable securities - restricted
Total marketable securities - restricted	29,456	29,330
Level 2 | Money market accounts
Cash equivalents
Total cash equivalents	0	0
Restricted cash equivalents
Total restricted cash equivalents	0	0
Level 2 | Certificates of deposits
Restricted cash equivalents
Total restricted cash equivalents	0	0
Level 2 | Corporate debt securities
Marketable securities - restricted
Total marketable securities - restricted	2,738	2,545
Level 2 | U.S. treasury and agency securities
Marketable securities - restricted
Total marketable securities - restricted	1,956	1,986
Level 2 | Commercial paper
Marketable securities - restricted
Total marketable securities - restricted	18,550	16,081
Level 2 | Asset backed securities
Marketable securities - restricted
Total marketable securities - restricted	6,212	8,718
Level 3
Cash equivalents
Total cash equivalents	0	0
Restricted cash equivalents
Total restricted cash equivalents	0	0
Marketable securities - restricted
Total marketable securities - restricted	0	0
Level 3 | Money market accounts
Cash equivalents
Total cash equivalents	0	0
Restricted cash equivalents
Total restricted cash equivalents	0	0
Level 3 | Certificates of deposits
Restricted cash equivalents
Total restricted cash equivalents	0	0
Level 3 | Corporate debt securities
Marketable securities - restricted
Total marketable securities - restricted	0	0
Level 3 | U.S. treasury and agency securities
Marketable securities - restricted
Total marketable securities - restricted	0	0
Level 3 | Commercial paper
Marketable securities - restricted
Total marketable securities - restricted	0	0
Level 3 | Asset backed securities
Marketable securities - restricted
Total marketable securities - restricted	$ 0	$ 0