Acquisitions - Purchase Price Allocation for Integrys Acquisition (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Jun. 29, 2015
Assets acquired
Goodwill		$ 0.0	$ 2,581.6
Integrys
Business Acquisition [Line Items]
Deferred taxes related to goodwill		0.0
Assets acquired
Current assets				$ 1,060.1
Property, plant, and equipment, net				7,107.4
Goodwill	$ 2,604.3
Other long-term assets				2,830.5
Liabilities assumed
Current liabilities				(1,320.7)
Long-term debt				(2,943.6)
Other long-term liabilities				(3,703.8)
Preferred stock of subsidiary				$ (51.1)
Total purchase price	$ 5,583.1
Accounting Standards Update 2015-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of adoption of ASU 2015-16		$ 0.0