CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net income	$ 208.7	$ 226.5	$ 161.2
Pension and other postretirement benefits
Change in unrecognized actuarial loss	25.7	(97.5)	167.7
Change in unrecognized prior service cost	(0.1)	0.6	1.1
Interest rate cash flow hedge amortization	0.6	0.5	0.5
Commodity cash flow hedge	(0.2)	0.0	0.0
Change in fair value of long-term investment	0.0	0.0	(0.1)
Income taxes	(10.0)	36.9	(64.7)
Net other comprehensive income (loss)	16.0	(59.5)	104.5
COMPREHENSIVE INCOME	224.7	167.0	265.7
Questar Pipeline [Member]
Net income	59.6	60.6	8.2
Pension and other postretirement benefits
Interest rate cash flow hedge amortization	0.6	0.5	0.5
Commodity cash flow hedge	(0.2)	0.0	0.0
Income taxes	(0.2)	(0.1)	(0.2)
Net other comprehensive income (loss)	0.2	0.4	0.3
COMPREHENSIVE INCOME	$ 59.8	$ 61.0	$ 8.5