Long-term debt: (Details) € in Millions, $ in Millions	1 Months Ended		3 Months Ended		6 Months Ended					9 Months Ended
	May 31, 2023 USD ($)	May 31, 2022 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($)	Apr. 30, 2023 USD ($)	Oct. 31, 2022 USD ($)	Jun. 30, 2022 USD ($)	Apr. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2022 EUR (€)
Interest rate swap
Long-term debt:
Interest expense			$ (1.3)	$ (7.5)	$ 0.5			$ (28.8)		$ 51.7
Interest rate swap | Cash
Long-term debt:
Margin deposits			51.7		51.7
Interest rate swap | Restricted cash
Long-term debt:
Margin deposits			51.6		51.6
Interest rate swap | Unrestricted cash
Long-term debt:
Margin deposits			0.1		$ 0.1
SOFR
Long-term debt:
Interest rate					3.50%
Senior secured 2026 Notes
Long-term debt:
Face amount			$ 500.0		$ 500.0
Interest rate (as a percent)			3.50%		3.50%
Senior notes outstanding			$ 500.0		$ 500.0
2027 Notes
Long-term debt:
Face amount			$ 450.0		$ 450.0
Interest rate (as a percent)			7.00%		7.00%
Senior secured notes due 2022 | Interest rate swap
Long-term debt:
Face amount			$ 51.6		$ 51.6
Net liability, prepaid expenses			23.3		23.3
Net liability, other long term			$ 28.2		$ 28.2
Senior unsecured 2024 Notes
Long-term debt:
Face amount | €											€ 350.0
2024 Notes
Long-term debt:
Interest rate (as a percent)											4.375%
Installment One | SOFR | Interest rate swap
Long-term debt:
Payment made to counter party	$ 9.5						$ 3.4
Installment Two | SOFR
Long-term debt:
Net proceeds									$ 1.2
Payment of debt in Installments		$ 1.2
Installment Two | SOFR | Interest rate swap
Long-term debt:
Net proceeds							$ 3.4
Installment Two | SOFR | Interest rate swap | Unrestricted cash
Long-term debt:
Net proceeds						$ 9.5