QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Effect Of Fourth Quarter Events [Line Items]
Operating revenue	$ 319.5	$ 90.9	$ 139.7	$ 396.9	$ 297.3	$ 87.9	$ 128.2	$ 407.9	$ 947.0	[1]	$ 921.3	[1]	$ 917.6	[1]
Income (loss) from operations	61.5	(6.6)	(2.5)	91.9	53.3	(19.8)	3.8	83.4	144.3		120.7		122.6
Net income (loss)	$ 30.0	$ (8.5)	(6.2)	$ 52.2	$ 28.9	(17.7)	$ (1.6)	$ 47.6	$ 67.5		$ 57.2		$ 64.3
Transaction Costs Associated with the Dominion Questar Combination | Dominion Questar Combination
Effect Of Fourth Quarter Events [Line Items]
Transaction costs, net of tax			$ 6.9			$ 7.7

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