Regulatory Assets and Liabilities - Regulatory Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities
Other current liabilities	$ 10.2	$ 0.0
Regulatory liabilities	853.9	741.2
Total regulatory liabilities	864.1	741.2
Removal costs
Regulatory Liabilities
Total regulatory liabilities	722.9	696.9
Mines deferral
Regulatory Liabilities
Total regulatory liabilities	70.2	31.6
Other, net
Regulatory Liabilities
Total regulatory liabilities	$ 71.0	$ 12.7