SIGNIFICANT ACCOUNTING POLICIES - Purchased Gas (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Purchased Gas [Abstract]
Gas purchases		$ 125.5	$ 102.0	$ 82.5
Affiliated operator service fee		304.2	311.7	319.0
Transportation and storage	[1]	84.9	79.3	79.2
Gathering		24.9	23.7	22.1
Royalties		21.6	26.3	33.3
Storage (injection), net		(3.6)	(5.5)	(3.5)
Purchased-gas account adjustment		(7.3)	(0.6)	20.5
Other		5.2	5.2	5.0
Total purchased gas	[1]	$ 555.4	$ 542.1	$ 558.1

[1] See Note 19 for amounts attributable to related parties.