Operating Segment - Schedule of Segment Information (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Operating revenue	[1]	$ 139.7	$ 128.2	$ 536.6	$ 536.1
Net income (loss)		(6.2)	(1.6)	46.0	46.0
Gas Infrastructure
Segment Reporting Information [Line Items]
Operating revenue		139.7	128.2	536.6	536.1
Net income (loss)		0.7	(1.6)	53.1	46.0
Corporate and Other
Segment Reporting Information [Line Items]
Operating revenue		0.0	0.0	0.0	0.0
Net income (loss)		$ (6.9)	$ 0.0	$ (7.1)	$ 0.0

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