Investments in Joint Ventures - Schedule of Results of Operations of the Joint Ventures (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Operating Revenue and Expenses
Revenues	$ 215.1	$ 163.8	$ 427.0	$ 318.9
Expenses	102.8	81.4	206.7	164.9
Excess of revenues over expenses	$ 112.3	$ 82.4	$ 220.3	$ 154.0