CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Net income (loss)	$ 33.8	$ 36.1	$ 148.2	$ 146.3	$ 209.8	$ 210.0
Other comprehensive income (loss):
Pension and other postretirement benefits	0	0	0	0	(159.9)	34.0
Interest rate cash flow hedges	0.2	(37.0)	0.4	(36.2)	(0.6)	(36.2)
Long-term investment	0.1	0	0.1	0	0.1	0
Income taxes	(0.2)	13.7	(0.3)	13.4	61.3	0.3
Net other comprehensive income (loss)	0.1	(23.3)	0.2	(22.8)	(99.1)	(1.9)
COMPREHENSIVE INCOME (LOSS)	33.9	12.8	148.4	123.5	110.7	208.1
Questar Pipeline [Member]
Net income (loss)	17.1	18.8	49.8	50.7	67.0	68.7
Other comprehensive income (loss):
Interest rate cash flow hedges	0.2	(37.0)	0.4	(36.2)	(0.6)	(36.2)
Income taxes	(0.1)	13.7	(0.2)	13.4	0.2	13.4
Net other comprehensive income (loss)	0.1	(23.3)	0.2	(22.8)	(0.4)	(22.8)
COMPREHENSIVE INCOME (LOSS)	$ 17.2	$ (4.5)	$ 50.0	$ 27.9	$ 66.6	$ 45.9