Long-term debt: Limitations under the Indentures (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($)
Long-term debt
Consolidated leverage ratio	4.25
Amount unrestricted and permitted for investment payments	$ 185.5
Fixed charge coverage ratio	2.0
Minimum
Long-term debt
Percentage Of Principal Amount By Debt Holders Entitled To Redeem Remaining 2024 Notes	90.00%
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
Consolidated leverage ratio	6.0
Consolidated secured leverage ratio	1.0
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	4.0
Fixed charge coverage ratio	1.0
Restriction on dividends and stock purchases | Senior unsecured 2024 Notes | Minimum
Long-term debt
Consolidated leverage ratio	1.00
Restriction on dividends and stock purchases | Senior unsecured 2024 Notes | Maximum
Long-term debt
Consolidated leverage ratio	4.25
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