Operating Segment - Schedule of Segment Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Operating revenue	[1]	$ 406.7	$ 396.9
Net income (loss)		59.1	52.2
Gas Infrastructure
Segment Reporting Information [Line Items]
Operating revenue		406.7	396.9
Net income (loss)		59.3	52.4
Corporate and Other
Segment Reporting Information [Line Items]
Operating revenue		0.0	0.0
Net income (loss)		$ (0.2)	$ (0.2)

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