Operating Segment - Schedule of Segment Information (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Operating revenue	[1]	$ 84.1	$ 90.9	$ 628.2	$ 627.5
Net loss		(11.6)	(8.5)	44.7	37.5
Gas Infrastructure
Segment Reporting Information [Line Items]
Operating revenue		84.1	90.9	628.2	627.5
Net loss		(11.5)	(7.0)	45.0	46.1
Corporate and Other
Segment Reporting Information [Line Items]
Operating revenue		0.0	0.0	0.0	0.0
Net loss		$ (0.1)	$ (1.5)	$ (0.3)	$ (8.6)

[1] See Note 13 for amounts attributable to related parties.