Long-term debt: Limitations under the Indentures (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Limitations under the Indentures
Consolidated leverage ratio	4.25
Permitted investments and payments	$ 64.3
Restriction on incurring additional indebtedness | Minimum
Limitations under the Indentures
Consolidated leverage ratio	5.0
Restriction on certain payments | Minimum
Limitations under the Indentures
Consolidated leverage ratio	4.25
Increase in unrestricted payment amount | Maximum
Limitations under the Indentures
Consolidated leverage ratio	4.25