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Long-term debt: - Senior unsecured notes (Details) € in Millions, $ in Millions		1 Months Ended		12 Months Ended
	Jun. 22, 2022	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2024 EUR (€)	Jun. 30, 2022 EUR (€)	Jun. 30, 2020 EUR (€)	Jun. 30, 2019 EUR (€)
Senior unsecured notes - €350.0 million 2024 Notes
Interest rate (as a percent)						5.00%
Redemption price percentage	101.094%
Senior Unsecured 2024 Notes
Senior unsecured notes - €350.0 million 2024 Notes
Principal amount | €						€ 350.0	€ 350.0	€ 215.0	€ 135.0
Net proceeds		$ 240.3	$ 152.1
Interest rate (as a percent)								4.375%
Debt fair value				$ 365.8	$ 397.0
Gain on foreign exchange				$ 31.6	$ 32.5
Redemption price percentage	101.094%

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