Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 17, 2015
Related Party Transaction [Line Items]
Letters of credit outstanding	$ 37.7	$ 29.4
Change in estimated value of letters of credit	(0.2)
Equity Method Investee
Related Party Transaction [Line Items]
Letters of credit outstanding	$ 1.4	$ 1.4	$ 2.1