Business Acquisitions and Dispositions Business Acquisitions and Dispositions - Aurora, Rock, Seward, Shelby (Details) $ in Millions							3 Months Ended			12 Months Ended
	Jul. 13, 2016 USD ($)	May 13, 2016 USD ($)	Mar. 02, 2016 USD ($)	Feb. 03, 2016 USD ($)	Nov. 25, 2015 USD ($)	Nov. 10, 2015 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) MW	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jul. 12, 2016 USD ($)	May 12, 2016 MW	Feb. 02, 2016 USD ($)	Nov. 24, 2015 MW	Nov. 09, 2015 MW
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset Impairment Charges										$ 918.0	$ 5,030.0	$ 97.0
Rockford [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of Ownership														100.00%
Proceeds from Sale of Property, Plant, and Equipment	$ 56.0	$ 55.0
Power Generation Capacity, Megawatts | MW														450
Asset Impairment Charges								$ 17.0
Base Residual Auction Results Adjustments													$ 1.0
Aurora Generating Station [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Property, Plant, and Equipment	$ 369.0	$ 365.0
Power Generation Capacity, Megawatts | MW								878
Base Residual Auction Results Adjustments													$ 4.0
Gain (Loss) on Disposition of Property Plant Equipment							$ 188.0
Seward Generating Station [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Property, Plant, and Equipment				$ 75.0	$ 75.0
Power Generation Capacity, Megawatts | MW																525
Asset Impairment Charges											$ 134.0
Disposal Group, Including Discontinued Operation, Cash															$ 3.0
Amount of Continuing Cash Flow After Disposition				5.0
Shelby County Energy Center, LLC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Property, Plant, and Equipment			$ 46.0			$ 46.0
Power Generation Capacity, Megawatts | MW																	352
Gain (Loss) on Disposition of Property Plant Equipment									$ 29.0
Future Revenue Rights									10.0
Receipt of Future Revenue Rights										$ 8.0
Annual [Member] | Seward Generating Station [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount of Continuing Cash Flow After Disposition				1.0
Environmental Testing [Member] | Seward Generating Station [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount of Continuing Cash Flow After Disposition				$ 2.5
Robindale Energy Services, Inc [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term Purchase Commitment, Amount									$ 13.0