Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of year	$ (246.3)
OCI before reclassifications	135.7
Reclassified from AOCI	33.5	[1]
Income taxes
Attributable to OCI before reclassifications	(51.8)
Attributable to amounts reclassified from AOCI	(12.9)	[2]
Income taxes	(64.7)		12.3	75.0
Net other comprehensive income	104.5		(20.2)	(122.1)
AOCI at end of year	(141.8)		(246.3)
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Income Tax Expense (Benefit)	(101.3)		(116.5)	(116.4)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Change in unrecognized actuarial loss			13.6	62.3
Change in unrecognized prior service cost			(1.0)	(1.1)
Change in fair value of long-term investment			(0.1)	0
Income taxes	(64.7)		12.3	75.0
Pension, Actuarial loss [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of year	(202.4)
OCI before reclassifications	123.4
Reclassified from AOCI	28.9	[1]
Income taxes
Attributable to OCI before reclassifications	(47.2)
Attributable to amounts reclassified from AOCI	(11.1)	[2]
Income taxes	(58.3)
Net other comprehensive income	94.0
AOCI at end of year	(108.4)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Income taxes	(58.3)
Pension, Prior service cost [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of year	(1.1)
OCI before reclassifications	0
Reclassified from AOCI	1.1	[1]
Income taxes
Attributable to OCI before reclassifications	0
Attributable to amounts reclassified from AOCI	(0.4)	[2]
Income taxes	(0.4)
Net other comprehensive income	0.7
AOCI at end of year	(0.4)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Income taxes	(0.4)
Pension, Total [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of year	(203.5)
OCI before reclassifications	123.4
Reclassified from AOCI	30.0	[1]
Income taxes
Attributable to OCI before reclassifications	(47.2)
Attributable to amounts reclassified from AOCI	(11.5)	[2]
Income taxes	(58.7)
Net other comprehensive income	94.7
AOCI at end of year	(108.8)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Income taxes	(58.7)
OPRB, Actuarial Loss [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of year	(19.7)
OCI before reclassifications	12.4
Reclassified from AOCI	3.0	[1]
Income taxes
Attributable to OCI before reclassifications	(4.7)
Attributable to amounts reclassified from AOCI	(1.2)	[2]
Income taxes	(5.9)
Net other comprehensive income	9.5
AOCI at end of year	(10.2)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Income taxes	(5.9)
Long-term investment [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of year	0.1
OCI before reclassifications	(0.1)
Reclassified from AOCI	0	[1]
Income taxes
Attributable to OCI before reclassifications	0.1
Attributable to amounts reclassified from AOCI	0	[2]
Income taxes	0.1
Net other comprehensive income	0
AOCI at end of year	0.1
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Income taxes	0.1
Questar Pipeline [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of year	(23.2)
Income taxes
Income taxes	(0.2)		(0.2)	13.8
Net other comprehensive income	0.3		0.2	(23.4)
AOCI at end of year	(22.9)		(23.2)
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Income Tax Expense (Benefit)	(7.0)		(37.3)	(38.2)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Interest rate cash flow hedges			(0.2)	13.8
Income taxes	(0.2)		(0.2)	13.8
Questar Pipeline [Member] | Interest rate cash flow hedges [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of year	(23.2)
OCI before reclassifications	0
Reclassified from AOCI	0.5	[1]
Income taxes
Attributable to OCI before reclassifications	0
Attributable to amounts reclassified from AOCI	(0.2)	[2]
Income taxes	(0.2)
Net other comprehensive income	0.3
AOCI at end of year	(22.9)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Income taxes	(0.2)
Questar Pipeline [Member] | Interest rate cash flow hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Interest Income (Expense), Net	(0.5)
Income Tax Expense (Benefit)	$ 0.2

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

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