Investments - Epic Gas (Details) - Epic Gas - USD ($) $ in Millions	3 Months Ended
Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Aug. 31, 2018	Nov. 30, 2017
Schedule of Equity Method Investments [Line Items]
Ownership percentage							21.10%	21.10%
Our investment in Epic Gas							$ 21.2	$ 22.2
Net losses	$ (2.3)	$ (2.7)	$ (16.4)	$ (5.4)	$ (3.4)	$ (15.9)

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