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Long-term debt: - Limitations under the Indentures (Details)	12 Months Ended
	Dec. 31, 2025 USD ($)
Long-term debt
Amount unrestricted and permitted for investment payments	$ 352,800,000
Indentures | Minimum
Long-term debt
Consolidated leverage ratio	1
Consolidated secured leverage ratio	1
Indebtedness that may occur	$ 1
Fixed charge coverage ratio	1
Indentures | Maximum
Long-term debt
Consolidated leverage ratio	6
Consolidated secured leverage ratio	4
Fixed charge coverage ratio	2
2027 Notes Indentures | Minimum
Long-term debt
Consolidated leverage ratio	1
Fixed charge coverage ratio	2
2027 Notes Indentures | Maximum
Long-term debt
Consolidated leverage ratio	6
Consolidated secured leverage ratio	4
2032 Notes Indenture | Minimum
Long-term debt
Consolidated leverage ratio	1
Fixed charge coverage ratio	2
2032 Notes Indenture | Maximum
Long-term debt
Consolidated leverage ratio	6
Consolidated secured leverage ratio	4

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