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Long-term debt: - Limitations under the Indentures (Details)	12 Months Ended
	Dec. 31, 2024 USD ($)
Long-term debt
Consolidated leverage ratio	6
Consolidated secured leverage ratio	4
Amount unrestricted and permitted for investment payments	$ 303,600,000
Fixed charge coverage ratio	2
Restriction on incurring additional indebtedness | Senior notes | Minimum
Long-term debt
Consolidated secured leverage ratio	1
Restriction on incurring additional indebtedness | Senior Unsecured 2024 Notes | Minimum
Long-term debt
Consolidated leverage ratio	1
Restriction on incurring additional indebtedness | Senior Unsecured 2024 Notes | Maximum
Long-term debt
Consolidated leverage ratio	6
Consolidated secured leverage ratio	4
Restriction on incurring additional indebtedness | Senior secured 2026 Notes | Minimum
Long-term debt
Consolidated secured leverage ratio	1.0
Fixed charge coverage ratio	2
Restriction on incurring additional indebtedness | Senior secured 2026 Notes | Maximum
Long-term debt
Consolidated secured leverage ratio	4.0
Fixed charge coverage ratio	1
Restriction on dividends and stock purchases | Senior Unsecured 2024 Notes | Minimum
Long-term debt
Indebtedness that may occur	$ 1
Restriction on dividends and stock purchases | Senior secured 2026 Notes
Long-term debt
Fixed charge coverage ratio	6
Restriction on dividends and stock purchases | Senior secured 2026 Notes | Minimum
Long-term debt
Consolidated leverage ratio	1
Fixed charge coverage ratio	1
Restriction on dividends and stock purchases | Senior secured 2026 Notes | Maximum
Long-term debt
Consolidated leverage ratio	2
Fixed charge coverage ratio	6

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