Long-term debt: Limitations under the Indentures (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($)
Long-term debt
Amount unrestricted and permitted for investment payments	$ 145.5
Restriction on incurring additional indebtedness | Senior unsecured 2024 Notes | Minimum
Long-term debt
Consolidated leverage ratio	6.0
Consolidated secured leverage ratio	4.25
Restriction on incurring additional indebtedness | Senior unsecured 2024 Notes | Maximum
Long-term debt
Consolidated leverage ratio	1.0
Consolidated secured leverage ratio	1.00
Restriction on incurring additional indebtedness | Senior secured 2026 Notes | Minimum
Long-term debt
Consolidated leverage ratio	6.0
Consolidated secured leverage ratio	4.0
Fixed charge coverage ratio	2.0
Restriction on incurring additional indebtedness | Senior secured 2026 Notes | Maximum
Long-term debt
Consolidated leverage ratio	1.0
Consolidated secured leverage ratio	1.0
Fixed charge coverage ratio	1.0
Restriction on dividends and stock purchases | Maximum
Long-term debt
Fixed charge coverage ratio	2.0
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	6.0
Fixed charge coverage ratio	2.0
Restriction on dividends and stock purchases | Senior secured 2026 Notes | Maximum
Long-term debt
Consolidated leverage ratio	1.0
Fixed charge coverage ratio	1.0
Increase in unrestricted payment amount | Maximum
Long-term debt
Consolidated leverage ratio	4.25