Related Party Transactions - Schedule of Related Party Expenses Including Personnel Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	[1]	Dec. 31, 2015	[1]
Related Party Transactions [Abstract]
Operations and maintenance - related parties	$ 45.6	$ 40.0		$ 39.0
General and administrative - related parties	$ 47.7	$ 43.7		$ 42.5

[1] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.