CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Net income	$ 84.6	$ 85.1	$ 226.0	$ 173.4
Other comprehensive income (loss):
Pension and other postretirement benefits	6.0	4.3	(95.2)	165.3
Interest rate cash flow hedge amortization	0.2	0.2	0.5	0.5
Change in fair value of long-term investment	0	0	0	(0.1)
Income taxes	(2.4)	(1.8)	36.3	(63.5)
Net other comprehensive income (loss)	3.8	2.7	(58.4)	102.2
COMPREHENSIVE INCOME	88.4	87.8	167.6	275.6
Questar Pipeline [Member]
Net income	13.9	15.7	58.8	8.1
Other comprehensive income (loss):
Interest rate cash flow hedge amortization	0.2	0.2	0.5	0.5
Income taxes	(0.1)	(0.1)	(0.1)	(0.2)
Net other comprehensive income (loss)	0.1	0.1	0.4	0.3
COMPREHENSIVE INCOME	$ 14.0	$ 15.8	$ 59.2	$ 8.4