Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015		Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances		$ (201.3)		$ (141.8)
OCI before reclassifications		3.6		(113.6)
Reclassified from AOCI	[2]	22.4	[1]	17.2
Income taxes
Attributable to OCI before reclassifications		(1.3)		43.4
Attributable to amounts reclassified from AOCI	[4]	(8.7)	[3]	(6.5)
Total income taxes		(10.0)		36.9	$ (64.7)
Net other comprehensive income (loss)		16.0		(59.5)	104.5
Ending balances		(185.3)		(201.3)	(141.8)
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Income tax (expense) benefit		(110.6)		(125.9)	(101.3)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes		(10.0)		36.9	(64.7)
Gas Domestic Regulated Revenue Transmission		187.9		190.2	189.5
Questar Pipeline [Member]
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Gas Domestic Regulated Revenue Transmission Affiliated Companies		0.4
Gas Domestic Regulated Revenue Transmission		1.0
Pension Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances		(167.8)		(108.8)
OCI before reclassifications		0.6		(111.5)
Reclassified from AOCI	[2]	22.0	[1]	16.0
Income taxes
Attributable to OCI before reclassifications		(0.2)		42.6
Attributable to amounts reclassified from AOCI	[4]	(8.4)	[3]	(6.1)
Total income taxes		(8.6)		36.5
Net other comprehensive income (loss)		14.0		(59.0)
Ending balances		(153.8)		(167.8)	(108.8)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes		(8.6)		36.5
Actuarial Loss [Member] | Pension Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances		(167.8)		(108.4)
OCI before reclassifications		0.6		(111.5)
Reclassified from AOCI	[2]	21.9	[1]	15.4
Income taxes
Attributable to OCI before reclassifications		(0.2)		42.6
Attributable to amounts reclassified from AOCI	[4]	(8.4)	[3]	(5.9)
Total income taxes		(8.6)		36.7
Net other comprehensive income (loss)		13.9		(59.4)
Ending balances		(153.9)		(167.8)	(108.4)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes		(8.6)		36.7
Actuarial Loss [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances		(11.1)		(10.2)
OCI before reclassifications		1.8		(2.1)
Reclassified from AOCI	[2]	1.2	[1]	0.7
Income taxes
Attributable to OCI before reclassifications		(0.7)		0.8
Attributable to amounts reclassified from AOCI	[4]	(0.5)	[3]	(0.3)
Total income taxes		(1.2)		0.5
Net other comprehensive income (loss)		1.8		(0.9)
Ending balances		(9.3)		(11.1)	(10.2)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes		(1.2)		0.5
Prior service cost [Member] | Pension Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances		0.0		(0.4)
OCI before reclassifications		0.0		0.0
Reclassified from AOCI	[2]	0.1	[1]	0.6
Income taxes
Attributable to OCI before reclassifications		0.0		0.0
Attributable to amounts reclassified from AOCI	[4]	0.0	[3]	(0.2)
Total income taxes		0.0		(0.2)
Net other comprehensive income (loss)		0.1		0.4
Ending balances		0.1		0.0	(0.4)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes		0.0		(0.2)
Long-term investment [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances		0.1		0.1
OCI before reclassifications		0.0		0.0
Reclassified from AOCI	[2]	0.0	[1]	0.0
Income taxes
Attributable to OCI before reclassifications		0.0		0.0
Attributable to amounts reclassified from AOCI	[4]	0.0	[3]	0.0
Total income taxes		0.0		0.0
Net other comprehensive income (loss)		0.0		0.0
Ending balances		0.1		0.1	0.1
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes		0.0		0.0
Questar Pipeline [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances		(22.5)
Income taxes
Total income taxes		(0.2)		(0.1)	(0.2)
Net other comprehensive income (loss)		0.2		0.4	0.3
Ending balances		(22.3)		(22.5)
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Income tax (expense) benefit		(33.4)		(35.6)	(7.0)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes		(0.2)		(0.1)	(0.2)
Gas Domestic Regulated Revenue Transmission Affiliated Companies		75.1		73.7	76.7
Gas Domestic Regulated Revenue Transmission		187.9		190.2	189.5
Questar Pipeline [Member] | Interest rate cash flow hedges [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances		(22.5)		(22.9)
OCI before reclassifications		0.0		0.0
Reclassified from AOCI	[2]	0.6	[1]	0.5
Income taxes
Attributable to OCI before reclassifications		0.0		0.0
Attributable to amounts reclassified from AOCI	[4]	(0.3)	[3]	(0.1)
Total income taxes		(0.3)		(0.1)
Net other comprehensive income (loss)		0.3		0.4
Ending balances		(22.2)		(22.5)	(22.9)
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes		(0.3)		(0.1)
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)		(0.6)		(0.5)
Income tax (expense) benefit		0.3		0.1
Questar Pipeline [Member] | Commodity cash flow hedges [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balances		0.0		0.0
OCI before reclassifications		1.2		0.0
Reclassified from AOCI	[2]	(1.4)	[1]	0.0
Income taxes
Attributable to OCI before reclassifications		(0.4)		0.0
Attributable to amounts reclassified from AOCI	[4]	0.5	[3]	0.0
Total income taxes		0.1		0.0
Net other comprehensive income (loss)		(0.1)		0.0
Ending balances		(0.1)		0.0	$ 0.0
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Total income taxes		0.1		0.0
Questar Pipeline [Member] | Commodity cash flow hedges [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comphrensive Income [Abstract]
Income tax (expense) benefit		(0.5)		0.0
Income taxes allocated to each component of other comprehensive income (loss) [Abstract]
Gas Domestic Regulated Revenue Transmission		$ 1.4		$ 0.0

[1] Commodity cash flow hedge amounts are included in their entirety as an increase in Questar Pipeline revenue on the Consolidated Statements of Income.

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

[3] Income tax reclassifications related to commodity cash flow hedge amounts are included in their entirety as charges to income taxes on the Consolidated Statements of Income.

[4] 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.