Acquisitions - Approval Conditions for Integrys Acquisition (Details) - Integrys - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Jun. 29, 2015
Earnings sharing mechanisms
Business Acquisition [Line Items]
Expense for earnings sharing mechanisms		$ 2.9	$ 24.4
WE
Business Acquisition [Line Items]
Duration of earnings cap condition imposed by the PSCW (in years)	3 years
Percentage of first 50 basis points of additional utility earnings shared with customers	50.00%
ROE in excess of authorized amount (as a percent)				0.50%
WG
Business Acquisition [Line Items]
Duration of earnings cap condition imposed by the PSCW (in years)	3 years
Percentage of first 50 basis points of additional utility earnings shared with customers	50.00%
ROE in excess of authorized amount (as a percent)				0.50%
PGL
Business Acquisition [Line Items]
Duration of base rate freeze condition imposed by the ICC (in years)	2 years
NSG
Business Acquisition [Line Items]
Duration of base rate freeze condition imposed by the ICC (in years)	2 years