Long-term debt: (Details) € in Millions						1 Months Ended		3 Months Ended			6 Months Ended
	Jun. 22, 2022 USD ($)	Jun. 22, 2022 EUR (€)	Dec. 01, 2021 USD ($)	May 07, 2021 USD ($)	Apr. 06, 2021 USD ($)	May 31, 2022 USD ($)	Mar. 31, 2021 USD ($)	Jun. 30, 2022 USD ($)	Oct. 31, 2021 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($)	Apr. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 EUR (€)
Long-term debt
Face amount | €														€ 350.0
Interest rate (as a percent)								4.375%			4.375%			4.375%
Margin deposits								$ 37,800,000			$ 37,800,000
Loss on debt extinguishment and redemption													$ 14,698,000
Redemption price percentage	101.094%	101.094%					103.20%
Unrealized foreign exchange gain (loss) on 2024 Euro Notes								23,547,000		$ (5,280,000)	31,561,000		13,590,000
Gain and loss on foreign exchange											(11,885,000)		(14,698,000)
Gain on foreign exchange											31,600,000		13,600,000
Interest Rate Swap
Long-term debt
Interest expense								7,500,000			28,800,000
Interest Rate Swap | Cash
Long-term debt
Margin deposits								48,500,000			48,500,000
Interest Rate Swap | Restricted Cash
Long-term debt
Margin deposits								48,500,000			48,500,000
Interest Rate Swap | Unrestricted Cash
Long-term debt
Margin deposits								10,700,000			$ 10,700,000
SOFR
Long-term debt
Interest rate											3.50%
Senior secured 2026 Notes
Long-term debt
Face amount				$ 500,000,000.0				$ 450,000,000.0			$ 450,000,000.0
Interest rate (as a percent)								7.00%			7.00%			7.00%
Net proceeds				496,900,000
Proceeds from Issuance of Secured Debt													$ 496,933,000
Discount on issuance of debt				1,800,000
Debt costs				$ 1,300,000							$ 1,317,000
Redemption price percentage of principal amount redeemed											100.00%
Long-term Debt, Gross								$ 500,000,000.0			$ 500,000,000.0
2027 Notes
Long-term debt
Face amount	$ 450,000,000.0
Proceeds from Issuance of Secured Debt											446,000,000.0
Discount on issuance of debt											2,700,000
Debt costs											1,300,000
Redemption price percentage of principal amount redeemed	100.00%	100.00%
Redemption price percentage	40.00%	40.00%
2027 Notes | Prior to June 15, 2024
Long-term debt
Redemption price percentage of principal amount redeemed	107.00%	107.00%
2027 Notes | June 15, 2024 to June 14, 2025
Long-term debt
Redemption price percentage of principal amount redeemed	103.50%	103.50%
Senior secured notes due 2022
Long-term debt
Face amount							$ 115,900,000
Interest Payable, Current			$ 11,500,000				400,000
Gain (Loss) on Repurchase of Debt Instrument			$ 10,800,000				$ 3,900,000
Redemption for debt, notice issued					$ 45,000,000.0
Redemption Value					$ 1,900,000
Redemption price per $1,000 aggregate principal amount					41.41533
Accrued Interest Value					$ 400,000
Accrued interest per $1,000 aggregate principal amount					9.70486
Extinguishment of Debt, Amount					$ 45,000,000.0
Long-term Debt, Gross					$ 284,100,000
Redemption price percentage							5.375%
Senior secured notes due 2022 | Interest Rate Swap
Long-term debt
Face amount								37,800,000			37,800,000
Net Liability, Prepaid expenses								9,700,000			9,700,000
Net liability, other long term								28,100,000			28,100,000
Senior unsecured Euro 2024 Notes
Long-term debt
Face amount								$ 500,000,000.0			$ 500,000,000.0
Interest rate (as a percent)								3.50%			3.50%			3.50%
Senior unsecured 2024 Notes
Long-term debt
Repayments of interest	$ 8,100,000	€ 7.7
Total repayment	383,400,000	361.5
Loss on debt extinguishment and redemption	11,900,000
Redemption Value	$ 375,200,000	€ 353.8
Aggregate Principal Amount Of Redemption | €		350.0
Redemption price percentage	101.094%	101.094%
2024 Notes
Long-term debt
Gain and loss on foreign exchange								$ 23,500,000		$ 5,300,000
Installment One | SOFR
Long-term debt
Net proceeds									$ 600,000
Payment of debt in Installments									$ 600,000
Installment Two | SOFR
Long-term debt
Net proceeds												$ 1,200,000
Payment of debt in Installments						$ 1,200,000