CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Net income	$ 78.5	$ 84.6	$ 202.6	$ 226.0
Other comprehensive income (loss):
Pension and other postretirement benefits	4.0	6.0	23.6	(95.2)
Interest rate cash flow hedge amortization	0.2	0.2	0.6	0.5
Commodity cash flow hedge	(0.2)	0.0	(0.4)	0.0
Income taxes	(1.6)	(2.4)	(9.2)	36.3
Net other comprehensive income (loss)	2.4	3.8	14.6	(58.4)
COMPREHENSIVE INCOME	80.9	88.4	217.2	167.6
Questar Pipeline [Member]
Net income	14.2	13.9	59.9	58.8
Other comprehensive income (loss):
Interest rate cash flow hedge amortization	0.2	0.2	0.6	0.5
Commodity cash flow hedge	(0.2)	0.0	(0.4)	0.0
Income taxes	(0.1)	(0.1)	(0.2)	(0.1)
Net other comprehensive income (loss)	(0.1)	0.1	0.0	0.4
COMPREHENSIVE INCOME	$ 14.1	$ 14.0	$ 59.9	$ 59.2