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Long-term debt: (Details) € in Millions		3 Months Ended			6 Months Ended
Long-term debt: (Details) € in Millions	Apr. 06, 2021 USD ($)	Mar. 31, 2022 USD ($)	Oct. 31, 2021 USD ($)	Mar. 31, 2021 USD ($)	Apr. 30, 2022 USD ($)	Mar. 31, 2022 EUR (€)
Long-term debt
Debt Instrument, Redemption Price, Percentage		103.20%
Principal amount plus accrued and unpaid interest (as a percent)		103.20%
Unrealized foreign exchange gain (loss) on 2024 Euro Notes		$ 8,014,000		$ 18,870,000
Restricted Cash
Long-term debt
Margin deposits		30,300,000
Unrestricted Cash
Long-term debt
Margin deposits		4,700,000
Interest Rate Swap
Long-term debt
Interest expense		21,300,000
Interest Rate Swap \| Cash
Long-term debt
Face amount		35,000,000.0
Interest expense		35,000,000.0
Margin deposits		$ 35,000,000.0
SOFR
Long-term debt
Interest rate		3.50%
Senior secured 2026 Notes
Long-term debt
Face amount		$ 500,000,000.0
Interest rate (as a percent)		3.50%				3.50%
Long-term Debt, Gross		$ 500,000,000.0
Senior secured notes due 2022
Long-term debt
Face amount		115,900,000
Repayments of interest	$ 11,500,000
Interest Payable, Current		400,000
Gain (Loss) on Repurchase of Debt Instrument		$ 3,900,000
Debt Instrument, Redemption Price, Percentage		5.375%
Redemption for debt, notice issued	45,000,000.0
Redemption Value	$ 1,900,000
Redemption price per $1,000 aggregate principal amount	41.41533
Aggregate Principal Amount Of Redemption	9.70486
Accrued interest per $1,000 aggregate principal amount	0.4
Extinguishment of Debt, Amount	$ 284,100,000
Long-term Debt, Gross	$ 10,800,000
Principal amount plus accrued and unpaid interest (as a percent)		5.375%
Senior secured notes due 2022 \| Interest Rate Swap
Long-term debt
Face amount		$ 30,300,000
Net Liability, Prepaid expenses		600,000
Net liability, other long term		29,700,000
Senior unsecured 2024 Notes
Long-term debt
Face amount		$ 389,000,000.0				€ 350.0
Interest rate (as a percent)		4.375%				4.375%
Long-term Debt, Gross		$ 389,000,000.0				€ 350.0
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