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, 2013		Dec. 31, 2012		Dec. 31, 2011
Revenues [Abstract]
From unaffiliated customers	$ 45.1		$ 36.1		$ 31.5
From affiliated company	294.8	[1]	274.1	[1]	253.6	[1]
Total revenues	339.9		310.2		285.1
Production costs	57.5		50.1		51.2
Depreciation, depletion and amortization	85.8		77.4		63.9
Total expenses	143.3		127.5		115.1
Revenues less expenses	196.6		182.7		170.0
Income taxes	(70.5)		(64.8)		(62.0)
Results of operations for cost-of-service gas- and oil-producing activities excluding corporate overhead and interest expenses	126.1		117.9		108.0
Reimbursement of general and administrative expenses from Questar Gas included in affiliated-company revenues	$ 27.5		$ 26.5		$ 23.5

[1] Primarily represents revenues received from Questar Gas pursuant to the Wexpro Agreement. Revenues include reimbursement of general and administrative expenses amounting to $27.5 million in 2013, $26.5 million in 2012 and $23.5 million in 2011.