Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		$ (193.8)	$ (136.3)	$ (201.3)	$ (141.8)	$ (133.8)
OCI before reclassifications		0.5		0.2
Reclassified from AOCI	[1]	6.8	3.8	19.2	12.9
Income taxes
OCI before reclassifications		(0.2)		(0.1)
Reclassified from AOCI	[2]	(2.7)	(1.3)	(7.4)	(4.9)
Total income taxes		(2.9)	(1.3)	(7.5)	(4.9)	34.3	$ (60.2)
Net other comprehensive income (loss)		4.4	2.5	11.9	8.0	(55.6)	96.9
AOCI at end of period		(189.4)	(133.8)	(189.4)	(133.8)	(189.4)	(133.8)
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Income tax benefit		16.2	17.5	86.4	90.1	122.2	132.0
Pension and OPRB
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		(171.4)	(113.7)	(178.9)	(119.0)	(111.3)
OCI before reclassifications		0.0		0.0
Reclassified from AOCI	[1]	6.7	3.7	18.7	12.5
Income taxes
OCI before reclassifications		0.0		0.0
Reclassified from AOCI	[2]	(2.6)	(1.3)	(7.1)	(4.8)
Total income taxes		(2.6)	(1.3)	(7.1)	(4.8)
Net other comprehensive income (loss)		4.1	2.4	11.6	7.7
AOCI at end of period		(167.3)	(111.3)	(167.3)	(111.3)	(167.3)	(111.3)
Long-term invest.
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		0.1	0.1	0.1	0.1	0.1
OCI before reclassifications		0.0		0.0
Reclassified from AOCI	[1]	0.0	0.0	0.0	0.0
Income taxes
OCI before reclassifications		0.0		0.0
Reclassified from AOCI	[2]	0.0	0.0	0.0	0.0
Total income taxes		0.0	0.0	0.0	0.0
Net other comprehensive income (loss)		0.0	0.0	0.0	0.0
AOCI at end of period		0.1	0.1	0.1	0.1	0.1	0.1
Questar Pipeline [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period				(22.5)		(22.6)
Income taxes
Total income taxes		(0.3)	0.0	(0.4)	(0.1)	(0.4)	(0.2)
Net other comprehensive income (loss)		0.3	0.1	0.3	0.3	0.4	0.3
AOCI at end of period		(22.2)	(22.6)	(22.2)	(22.6)	(22.2)	(22.6)
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Income tax benefit		8.0	8.6	24.8	26.8	33.6	36.0
Questar Pipeline [Member] | Interest rate cash flow hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		(22.3)	(22.7)	(22.5)	(22.9)	(22.6)
OCI before reclassifications		0.0		0.0
Reclassified from AOCI	[1]	0.1	0.1	0.5	0.4
Income taxes
OCI before reclassifications		0.0		0.0
Reclassified from AOCI	[2]	(0.1)	0.0	(0.3)	(0.1)
Total income taxes		(0.1)	0.0	(0.3)	(0.1)
Net other comprehensive income (loss)		0.0	0.1	0.2	0.3
AOCI at end of period		(22.3)	(22.6)	(22.3)	(22.6)	(22.3)	(22.6)
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)		0.1	0.1	0.5	0.4	0.6	0.5
Income tax benefit		(0.1)	$ 0.0	(0.3)	$ (0.1)	(0.3)	$ (0.2)
Questar Pipeline [Member] | Commodity cash flow hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		(0.2)		0.0
OCI before reclassifications		0.5		0.2
Reclassified from AOCI	[1]	0.0		0.0
Income taxes
OCI before reclassifications		(0.2)		(0.1)
Reclassified from AOCI	[2]	0.0		0.0
Total income taxes		(0.2)		(0.1)
Net other comprehensive income (loss)		0.3		0.1
AOCI at end of period		$ 0.1		$ 0.1		$ 0.1

[1] Interest rate cash flow hedge amounts are included in their entirety as charges to interest expense on the Consolidated Statements of Income.

[2] Income tax reclassifications related to interest rate cash flow hedge amounts are included in their entirety as credits to income taxes on the Consolidated Statements of Income.