CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net income	$ 32.6	$ 38.6	$ 157.8	$ 164.0	$ 220.3	$ 232.1
Other comprehensive income (loss):
Pension and other postretirement benefits	6.7	3.7	18.7	12.5	(90.7)	156.6
Interest rate cash flow hedge amortization	0.1	0.1	0.5	0.4	0.6	0.5
Commodity cash flow hedge	0.5	0.0	0.2	0.0	0.2	0.0
Income taxes	(2.9)	(1.3)	(7.5)	(4.9)	34.3	(60.2)
Net other comprehensive income (loss)	4.4	2.5	11.9	8.0	(55.6)	96.9
COMPREHENSIVE INCOME	37.0	41.1	169.7	172.0	164.7	329.0
Questar Pipeline [Member]
Net income	15.2	14.9	44.3	46.3	58.6	62.1
Other comprehensive income (loss):
Interest rate cash flow hedge amortization	0.1	0.1	0.5	0.4	0.6	0.5
Commodity cash flow hedge	0.5	0.0	0.2	0.0	0.2	0.0
Income taxes	(0.3)	0.0	(0.4)	(0.1)	(0.4)	(0.2)
Net other comprehensive income (loss)	0.3	0.1	0.3	0.3	0.4	0.3
COMPREHENSIVE INCOME	$ 15.5	$ 15.0	$ 44.6	$ 46.6	$ 59.0	$ 62.4