Regulatory Matters - Summary of Utilities Rate Plans (RECO) (Details) - RECO - USD ($) $ in Millions	1 Months Ended	12 Months Ended	31 Months Ended
	Mar. 31, 2017	Dec. 31, 2017	Feb. 28, 2017
Public Utilities, General Disclosures [Line Items]
Amortization to income of net regulatory (assets) and liabilities	$ 0.2		$ 0.4
Regulatory liabilities, amortization period	3 years		3 years
Amortization to income of net regulatory assets	$ (25.6)		$ (25.6)
Regulatory assets, amortization period	4 years		4 years
Average rate base	$ 178.7		$ 172.2
Weighted average cost of capital (after-tax) (percent)	7.47%		7.83%
Authorized return on common equity (percent)	9.60%		9.75%
Cost of long-term debt (percent)	5.37%		5.89%
Common equity ratio (percent)	49.70%		50.00%
Electric
Public Utilities, General Disclosures [Line Items]
Plan electric system storm hardening amount		$ 15.7
Plan period (years)		3 years
Year 1
Public Utilities, General Disclosures [Line Items]
Base rate changes	$ 1.7		$ 13.0
Actual return on common equity (percent)			9.20%
Year 2
Public Utilities, General Disclosures [Line Items]
Actual return on common equity (percent)			8.70%