CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Net income	$ 40.6	$ 40.3	$ 125.2	$ 125.4	$ 226.3	$ 174.3
Other comprehensive income (loss):
Pension and other postretirement benefits	6.0	4.5	12.0	8.8	(93.7)	161.9
Interest rate cash flow hedge amortization	0.2	0.1	0.4	0.3	0.6	0.5
Commodity cash flow hedge	(0.3)	0.0	(0.3)	0.0	(0.3)	0.0
Income taxes	(2.2)	(1.8)	(4.6)	(3.6)	35.9	(62.3)
Net other comprehensive income (loss)	3.7	2.8	7.5	5.5	(57.5)	100.1
COMPREHENSIVE INCOME	44.3	43.1	132.7	130.9	168.8	274.4
Questar Pipeline [Member]
Net income	15.2	15.7	29.1	31.4	58.3	9.3
Other comprehensive income (loss):
Interest rate cash flow hedge amortization	0.2	0.1	0.4	0.3	0.6	0.5
Commodity cash flow hedge	(0.3)	0.0	(0.3)	0.0	(0.3)	0.0
Income taxes	0.0	0.0	(0.1)	(0.1)	(0.1)	(0.2)
Net other comprehensive income (loss)	(0.1)	0.1	0.0	0.2	0.2	0.3
COMPREHENSIVE INCOME	$ 15.1	$ 15.8	$ 29.1	$ 31.6	$ 58.5	$ 9.6