Long-term debt: Limitations under the Indentures (Details)	6 Months Ended
	Jun. 30, 2023 USD ($)
Long-term debt:
Consolidated leverage ratio	6.0
Consolidated secured leverage ratio	4.0
Amount unrestricted and permitted for investment payments	$ 391,500,000
Fixed charge coverage ratio	2.0
Restriction on incurring additional indebtedness | Senior notes | Minimum
Long-term debt:
Consolidated secured leverage ratio	1.0
Restriction on incurring additional indebtedness | Senior unsecured 2024 Notes | Minimum
Long-term debt:
Consolidated leverage ratio	1.0
Restriction on incurring additional indebtedness | Senior unsecured 2024 Notes | Maximum
Long-term debt:
Consolidated leverage ratio	6.0
Consolidated secured leverage ratio	4.0
Restriction on incurring additional indebtedness | Senior secured 2026 Notes | Minimum
Long-term debt:
Fixed charge coverage ratio	2.0
Restriction on incurring additional indebtedness | Senior secured 2026 Notes | Maximum
Long-term debt:
Fixed charge coverage ratio	1.0
Restriction on dividends and stock purchases | Senior secured 2026 Notes
Long-term debt:
Fixed charge coverage ratio	1
Restriction on dividends and stock purchases | Senior secured 2026 Notes | Minimum
Long-term debt:
Consolidated leverage ratio	1.0
Fixed charge coverage ratio	1.0
Restriction on dividends and stock purchases | Senior secured 2026 Notes | Maximum
Long-term debt:
Consolidated leverage ratio	6.0
Fixed charge coverage ratio	2.0
Unrestricted general basket payment
Long-term debt:
Amount unrestricted and permitted for investment payments	$ 250,000.0