Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period	$ (246.3)		$ (226.1)
OCI before reclassifications	0		(32.9)
Reclassified from AOCI	0.2		0.5
Income taxes
Attributable to OCI before reclassifications	0		12.5
Attributable to amounts reclassified from AOCI	(0.1)		(0.2)
Total income taxes	(0.1)	(0.1)	12.3	74.9
Net other comprehensive income (loss)	0.1	0	(20.1)	(122.1)
AOCI at March 31, 2013	(246.2)	(226.1)	(246.2)	(226.1)
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Income tax benefit (expense)	(47.9)	(43.3)	(121.1)	(119.0)
Pension and OPRB
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period	(223.2)		(202.8)
OCI before reclassifications	0		(33.0)
Reclassified from AOCI	0		0
Income taxes
Attributable to OCI before reclassifications	0		12.6
Attributable to amounts reclassified from AOCI	0		0
Total income taxes	0		12.6
Net other comprehensive income (loss)	0		(20.4)
AOCI at March 31, 2013	(223.2)		(223.2)
Long-term invest.
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period	0.1		0.1
OCI before reclassifications	0		0.1
Reclassified from AOCI	0		0
Income taxes
Attributable to OCI before reclassifications	0		(0.1)
Attributable to amounts reclassified from AOCI	0		0
Total income taxes	0		(0.1)
Net other comprehensive income (loss)	0		0
AOCI at March 31, 2013	0.1		0.1
Questar Pipeline [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period	(23.2)		(23.4)
Income taxes
Total income taxes	(0.1)	(0.1)	(0.2)	13.7
Net other comprehensive income (loss)	0.1	0	0.3	(23.4)
AOCI at March 31, 2013	(23.1)	(23.4)	(23.1)	(23.4)
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Income tax benefit (expense)	(9.1)	(9.5)	(36.9)	(39.0)
Questar Pipeline [Member] | Interest rate cash flow hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period	(23.2)		(23.4)
OCI before reclassifications	0		0
Reclassified from AOCI	0.2		0.5
Income taxes
Attributable to OCI before reclassifications	0		0
Attributable to amounts reclassified from AOCI	(0.1)		(0.2)
Total income taxes	(0.1)		(0.2)
Net other comprehensive income (loss)	0.1		0.3
AOCI at March 31, 2013	(23.1)		(23.1)
Questar Pipeline [Member] | Interest rate cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Interest (expense)	(0.2)		(0.5)
Income tax benefit (expense)	$ 0.1		$ 0.2