Variable Interest Entities (Details) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) MW	Sep. 30, 2014 USD ($)	Jun. 29, 2015	Dec. 31, 2014 USD ($)
Variable interest entities
Equity method investment, ownership interest (as a percent)			60.00%
Equity investment in ATC	$ 999.4			$ 424.1
Purchased power agreement
Variable interest entities
Firm capacity from purchased power agreement (in megawatts) | MW	236
Minimum energy requirements over the remaining term of the purchased power agreement (in megawatts) | MW	0
Remaining term of the purchased power agreement (in years)	7 years
Residual guarantee associated with the purchased power agreement	$ 0.0
Required payments over the remaining term of the purchased power agreement	141.3
Total capacity and lease payments	$ 40.2	$ 39.8
ATC
Variable interest entities
Equity method investment, ownership interest (as a percent)	60.00%
Equity investment in ATC	$ 999.4
Accounts payable due to ATC	$ 28.1