Operating Revenue (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Public Utilities, General Disclosures [Line Items]
Service		$ 1.0	$ 1.0	$ 3.8	$ 3.7
Other		5.0	4.5	12.5	14.8
Total Revenues	[1]	87.9	89.3	624.0	605.8
Residential and commercial
Public Utilities, General Disclosures [Line Items]
Residential, commercial, and industrial sales		72.1	73.8	576.7	554.9
Industrial
Public Utilities, General Disclosures [Line Items]
Residential, commercial, and industrial sales		3.5	4.5	13.0	16.9
Transportation for industrial customers		$ 6.3	$ 5.5	$ 18.0	$ 15.5

[1] See Note 9 for amounts attributable to affiliates