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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating Revenue and Expenses
Revenues	$ 271.8	$ 231.2	$ 537.9	$ 457.0
Expenses	149.4	117.6	294.7	258.2
Excess of revenues over expenses	$ 122.4	$ 113.6	$ 243.2	$ 198.8