Long-term debt: Limitations under the Indentures (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)
Long-term debt
Amount permitted for investment payments	$ 165.5
Restriction on incurring additional indebtedness | Senior secured 2022 notes | Minimum
Long-term debt
Consolidated secured leverage ratio	4.0
Restriction on incurring additional indebtedness | Senior unsecured 2024 notes | Minimum
Long-term debt
Consolidated leverage ratio	6.0
Consolidated secured leverage ratio	3.5
Restriction on incurring additional indebtedness | Senior secured 2022 Notes and 2021 Notes | Minimum
Long-term debt
Consolidated leverage ratio	5.0
Consolidated secured leverage ratio	4.25
Restriction on dividends and stock purchases | Minimum
Long-term debt
Consolidated leverage ratio	4.25
Increase in unrestricted payment amount | Maximum
Long-term debt
Consolidated leverage ratio	4.25