Discontinued Operations and Dispositions - Disposition of Majority Interest in EVgo (Details) $ in Millions		3 Months Ended		6 Months Ended
	Jun. 17, 2016 USD ($) charging_station parking_space	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss on sale		$ (2.0)	$ 83.0	$ (4.0)	$ 83.0
Equity method investments		1,127.0		$ 1,127.0		$ 1,120.0
EVgo | Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration	$ 39.0
Cash received	17.0
Working capital adjustments	2.5
Capital contributions	15.0
Future contributions	7.0
Future earnout potential	70.0
Original financial obligation	$ 102.5
Number of public fast charging Freedom Station sites | charging_station	200
Number of commercial and multi-family parking spaces | parking_space	10,000
Loss on sale			83.0
Loss on the sale of the equity interest			$ 27.0
Remaining obligation	$ 56.0
Remaining interest				35.00%
Equity method investments		$ 3.0		$ 3.0