Long-term debt - Narrative (Details) - USD ($)		3 Months Ended		6 Months Ended
	Nov. 23, 2020	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021	Sep. 28, 2020	Sep. 22, 2020
Debt Instrument [Line Items]
Debt issuance costs								$ 3,100,000
Debt issuance cost, amount paid to subsidiary								1,900,000
Debt issuance costs capitalized								$ 1,200,000
5.50% Senior Secured Notes
Debt Instrument [Line Items]
Long-term debt, gross		$ 125,000,000		$ 125,000,000		$ 125,000,000
Interest expense		$ 1,700,000	$ 1,700,000	$ 3,400,000	$ 3,400,000
6.75% Senior Secured Notes
Debt Instrument [Line Items]
Long-term debt, gross							$ 150,000,000
Long term debt held in treasury							1,400,000
Long term debt outstanding, net of amount held in treasury							148,600,000
Senior Secured Notes | 5.50% Senior Secured Notes
Debt Instrument [Line Items]
Stated interest rate (as a percent)								5.50%
Aggregate principal amount of debt issued								$ 125,000,000
Issue price (as a percent)								100.00%
Redemption price (as a percent)	99.80%
Interest rate during period (as a percent)		5.50%		5.50%
Senior Secured Notes | 6.75% Senior Secured Notes
Debt Instrument [Line Items]
Stated interest rate (as a percent)								6.75%
Long-term debt, gross							$ 150,000,000