QUARTERLY FINANCIAL DATA (UNAUDITED) (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
Effect of Fourth Quarter Events [Line Items]
Operating revenue	$ 297.3	$ 87.9	$ 128.2	$ 407.9	$ 311.8	$ 89.3	$ 141.7	$ 374.8	$ 921.3	[1]	$ 917.6	[1]	$ 960.9	[1]
Income (loss) from operations	53.3	(19.8)	3.8	83.4	53.5	(8.9)	1.4	76.6	120.7		122.6		109.5
Net income (loss)	$ 28.9	(17.7)	$ (1.6)	$ 47.6	$ 32.1	$ (8.8)	$ (2.8)	$ 43.8	$ 57.2		$ 64.3		$ 55.2
Transaction Costs Associated with the Dominion Questar Combination | Affiliated Entity
Effect of Fourth Quarter Events [Line Items]
Transaction costs, net of tax		$ 7.7

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