Long-term debt - Narrative (Details) - USD ($)				3 Months Ended	12 Months Ended
	Nov. 23, 2020	Sep. 28, 2020	Aug. 25, 2019	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Sep. 22, 2020	Jun. 23, 2017
Debt Instrument [Line Items]
Long term debt outstanding, net of amount held in treasury		$ 148,600,000
Debt issuance cost, Amount paid to Subsidiary					$ 1,900,000
Debt issuance costs capitalized					1,200,000
Debt issuance costs					3,100,000
Payment for Debt Extinguishment or Debt Prepayment Cost		2,500,000
Write-off of debt issuance costs		341,200			0	$ 341,000	$ 184,000
Gain on repurchase of senior secured notes					0	86,000	0
Cash paid during the year for interest					10,089,000	19,013,000	40,678,000
5.50% Senior Secured Notes
Debt Instrument [Line Items]
Long-term debt gross					125,000,000	125,000,000
Interest Expense, Debt					6,900,000	1,900,000
Cash paid during the year for interest					$ 7,000,000	0
5.50% Senior Secured Notes | Senior Secured Notes
Debt Instrument [Line Items]
Aggregate principal amount of debt issued								$ 125,000,000
Stated interest rate (as a percent)								5.50%
Issue price (as a percent)								100.00%
Redemption price (as a percent)	99.80%
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
Interest Expense, Debt						7,400,000	$ 12,300,000
Interest Paid Long Term Debt						7,400,000
Debt Instrument, Repurchase Amount				$ 1,400,000
Gain on repurchase of senior secured notes				$ 85,560
6.75% Senior Secured Notes | Senior Secured Notes
Debt Instrument [Line Items]
Aggregate principal amount of debt issued									$ 200,000,000
Stated interest rate (as a percent)								6.75%	6.75%
Issue price (as a percent)									100.00%
Long-term debt gross						$ 150,000,000
Redemption price (as a percent)		101.6875%	25.00%
Repayments of Secured Debt			$ 50,000,000
Total Debt Redemption Costs on 25% Note Redemption			1,900,000
Payment for Debt Extinguishment or Debt Prepayment Cost			1,700,000
Write-off of debt issuance costs			$ 200,000