OPERATING REVENUE (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Public Utilities, General Disclosures [Line Items]
Gas transportation										$ 24.6		$ 21.2		$ 17.9
Other	[1]									24.8		25.5		37.4
Total operating revenue		$ 297.3	$ 87.9	$ 128.2	$ 407.9	$ 311.8	$ 89.3	$ 141.7	$ 374.8	921.3	[2]	917.6	[2]	960.9	[2]
Residential and commercial
Public Utilities, General Disclosures [Line Items]
Residential, commercial, and industrial sales										854.6		847.3		875.7
Industrial
Public Utilities, General Disclosures [Line Items]
Residential, commercial, and industrial sales										$ 17.3		$ 23.6		$ 29.9

[1] See Note 18 for amounts attributable to affiliates

[2] See Note 18 for amounts attributable to related parties.