Significant Accounting Policies - Concentration of Credit Risk (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Significant Accounting Policies
Allowance for doubtful accounts receivable	$ 0.7	$ 0.7
Maturity period of irrevocable letters of credit, minimum	0 days
Maturity period of irrevocable letters of credit, maximum	90 days