Operating Segment - Schedule of Segment Information (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Operating revenue	[1]	$ 90.9	$ 87.9	$ 627.5	$ 624.0
Net income (loss)		(8.5)	(17.7)	37.5	28.3
Gas Infrastructure
Segment Reporting Information [Line Items]
Operating revenue		90.9	87.9	627.5	624.0
Net income (loss)		(7.0)	(10.0)	46.1	36.0
Corporate and Other
Segment Reporting Information [Line Items]
Operating revenue		0.0	0.0	0.0	0.0
Net income (loss)		$ (1.5)	$ (7.7)	$ (8.6)	$ (7.7)

[1] See Note 12 for amounts attributable to related parties.