CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Net income (loss)	$ 38.6	$ (19.2)	$ 164.0	$ 93.1	$ 232.1	$ 156.9
Other comprehensive income (loss):
Pension and other postretirement benefits	3.7	9.0	12.5	24.7	156.6	(28.1)
Interest rate cash flow hedge amortization	0.1	0.1	0.4	0.4	0.5	0.4
Change in fair value of long-term investment	0	0	0	(0.1)	0	(0.1)
Income taxes	(1.3)	(3.4)	(4.9)	(9.4)	(60.2)	10.8
Net other comprehensive income (loss)	2.5	5.7	8.0	15.6	96.9	(17.0)
COMPREHENSIVE INCOME (LOSS)	41.1	(13.5)	172.0	108.7	329.0	139.9
Questar Pipeline [Member]
Net income (loss)	14.9	(37.9)	46.3	(7.6)	62.1	7.3
Other comprehensive income (loss):
Interest rate cash flow hedge amortization	0.1	0.1	0.4	0.4	0.5	0.4
Income taxes	0	0	(0.1)	(0.1)	(0.2)	(0.1)
Net other comprehensive income (loss)	0.1	0.1	0.3	0.3	0.3	0.3
COMPREHENSIVE INCOME (LOSS)	$ 15.0	$ (37.8)	$ 46.6	$ (7.3)	$ 62.4	$ 7.6