CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net income (loss)	$ (19.2)	$ 33.8	$ 93.1	$ 148.2	$ 156.9	$ 209.8
Other comprehensive income (loss):
Pension and other postretirement benefits	9.0	6.6	24.7	19.8	(28.1)	(150.6)
Interest rate cash flow hedges	0.1	0.2	0.4	0.4	0.4	(0.6)
Change in fair value of long-term investment	0	0.1	(0.1)	0.1	(0.1)	0.1
Income taxes	(3.4)	(2.8)	(9.4)	(7.9)	10.8	57.7
Net other comprehensive income (loss)	5.7	4.1	15.6	12.4	(17.0)	(93.4)
COMPREHENSIVE INCOME (LOSS)	(13.5)	37.9	108.7	160.6	139.9	116.4
Questar Pipeline [Member]
Net income (loss)	(37.9)	17.1	(7.6)	49.8	7.3	67.0
Other comprehensive income (loss):
Interest rate cash flow hedges	0.1	0.2	0.4	0.4	0.4	(0.6)
Income taxes	0	(0.1)	(0.1)	(0.2)	(0.1)	0.2
Net other comprehensive income (loss)	0.1	0.1	0.3	0.2	0.3	(0.4)
COMPREHENSIVE INCOME (LOSS)	$ (37.8)	$ 17.2	$ (7.3)	$ 50.0	$ 7.6	$ 66.6