Supplemental Gas and Oil Information - Unaudited Results of Operations for Oil and Gas Producing Activities (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues [Abstract]
From unaffiliated customers		$ 22.9	$ 35.6	$ 45.1
From affiliated company	[1]	319.1	350.3	294.8
Total revenues		342.0	385.9	339.9
Production costs		50.1	67.5	57.5
Exploration expenses		0.7	1.6	0.0
Depreciation, depletion and amortization		99.4	100.5	85.8
Abandonment and impairment		12.5	2.0	0.0
Total expenses		162.7	171.6	143.3
Revenues less expenses		179.3	214.3	196.6
Income taxes		(59.6)	(71.4)	(70.5)
Results of operations for gas- and oil-producing activities (excluding corporate overhead and interest costs)		119.7	142.9	126.1
Reimbursement of general and administrative expenses from Questar Gas included in affiliated-company revenues		$ 28.9	$ 30.5	$ 27.5

[1] Primarily represents revenues received from Questar Gas pursuant to the Wexpro agreements. Revenues include reimbursement of general and administrative expenses amounting to $28.9 million in 2015, $30.5 million in 2014 and $27.5 million in 2013.