Regulatory Assets and Liabilities - Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities
Other current liabilities	$ 33.4	$ 33.8
Regulatory liabilities	1,563.8	1,392.2
Total regulatory liabilities	1,597.2	1,426.0
Removal costs
Regulatory Liabilities
Total regulatory liabilities	1,262.7	1,209.6
Mines deferral
Regulatory Liabilities
Total regulatory liabilities	70.2	31.6
Energy costs refundable through rate adjustments
Regulatory Liabilities
Total regulatory liabilities	88.7	76.9
Unrecognized pension and OPEB costs
Regulatory Liabilities
Total regulatory liabilities	63.0	26.3
Derivatives
Regulatory Liabilities
Total regulatory liabilities	41.1	12.6
Uncollectible expense
Regulatory Liabilities
Total regulatory liabilities	36.1	31.8
Other, net
Regulatory Liabilities
Total regulatory liabilities	$ 35.4	$ 37.2