Supplemental Gas and Oil Information - Unaudited Results of Operations for Oil and Gas Producing Activities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Revenues [Abstract]
From unaffiliated customers	$ 35.6		$ 45.1		$ 36.1
From affiliated company	350.3	[1]	294.8	[1]	274.1	[1]
Total revenues	385.9		339.9		310.2
Production costs	67.5		57.5		50.1
Exploration expenses	1.6		0		0
Depreciation, depletion and amortization	100.5		85.8		77.4
Abandonment and impairment	2.0		0		0
Total expenses	171.6		143.3		127.5
Revenues less expenses	214.3		196.6		182.7
Income taxes	(71.4)		(70.5)		(64.8)
Results of operations for gas- and oil-producing activities (excluding corporate overhead and interest costs)	142.9		126.1		117.9
Reimbursement of general and administrative expenses from Questar Gas included in affiliated-company revenues	$ 30.5		$ 27.5		$ 26.5

[1] Primarily represents revenues received from Questar Gas pursuant to the Wexpro agreements. Revenues include reimbursement of general and administrative expenses amounting to $30.5 million in 2014, $27.5 million in 2013 and $26.5 million in 2012.