Long-term debt: Limitations under the Indentures (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)
Limitations under the Indentures
Permitted investments and payments	$ 44.0
Maximum
Limitations under the Indentures
Permitted investments and payments	43.2
Holdings
Limitations under the Indentures
Permitted investments and payments	$ 0.8
Senior Unsecured Notes | Minimum
Limitations under the Indentures
Consolidated leverage ratio to restrict on incurring additional indebtedness	5.0
Senior Unsecured Notes | Maximum
Limitations under the Indentures
Consolidated leverage ratio to be maintained to increase amount of permitted investments and payments by consolidated cash flow	4.25