Variable Interest Entities (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) MW	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
ATC
Variable interest entities
Equity method investment, ownership interest (as a percent)		60.00%
Equity investment in ATC		$ 1,443.9	$ 1,380.9
Accounts payable to related parties		$ 28.7	28.3
Purchased power agreement
Variable interest entities
Firm capacity from purchased power agreement (in megawatts) | MW		236
Minimum energy requirements over remaining term of purchased power agreement (in megawatts) | MW		0
Remaining term of purchased power agreement (in years)		5 years
Residual guarantee associated with purchased power agreement		$ 0.0
Required payments over remaining term of purchased power agreement		85.3
Total capacity and lease payments under purchased power agreement		$ 54.2	$ 53.6	$ 53.0
Accounting Standards Update 2015-02
Variable interest entities
Changes to disclosures and financial statement presentation	$ 0.0