Operating Segment - Schedule of Segment Information (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Operating revenue	[1]	$ 137.4	$ 139.7	$ 544.1	$ 536.6
Net income (loss)		(2.8)	(6.2)	56.3	46.0
Gas Infrastructure
Segment Reporting Information [Line Items]
Operating revenue		137.4	139.7	544.1	536.6
Net income (loss)		(2.8)	0.7	56.5	53.1
Corporate and Other
Segment Reporting Information [Line Items]
Operating revenue		0.0	0.0	0.0	0.0
Net income (loss)		$ 0.0	$ (6.9)	$ (0.2)	$ (7.1)

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