CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Net income	$ 72.9	$ 75.2	$ 209.7	$ 213.2
Other comprehensive income (loss):
Pension and other postretirement benefits	0	0	(33.0)	(159.9)
Interest rate cash flow hedges	0.2	0.1	0.5	(37.1)
Change in fair value of long-term investment	0	0	0.1	0
Income taxes	(0.1)	(0.1)	12.3	74.9
Net other comprehensive income (loss)	0.1	0	(20.1)	(122.1)
COMPREHENSIVE INCOME	73.0	75.2	189.6	91.1
Questar Pipeline [Member]
Net income	15.8	16.6	63.9	69.2
Other comprehensive income (loss):
Interest rate cash flow hedges	0.2	0.1	0.5	(37.1)
Income taxes	(0.1)	(0.1)	(0.2)	13.7
Net other comprehensive income (loss)	0.1	0	0.3	(23.4)
COMPREHENSIVE INCOME	$ 15.9	$ 16.6	$ 64.2	$ 45.8