Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		$ (182.9)	$ (197.5)	$ (185.3)	$ (201.3)	$ (193.8)
OCI before reclassifications		(0.9)	(0.3)	(1.1)	(0.3)
Reclassified from AOCI	[1]	4.1	6.2	8.3	12.4
Income taxes
Attributable to OCI before reclassifications		0.4	0.1	0.4	0.1
Reclassified from AOCI	[2]	(1.5)	(2.3)	(3.1)	(4.7)
Total income taxes		(1.1)	(2.2)	(2.7)	(4.6)	(8.1)	$ 35.9
Net other comprehensive income (loss)		2.1	3.7	4.5	7.5	13.0	(57.5)
AOCI at end of period		(180.8)	(193.8)	(180.8)	(193.8)	(180.8)	(193.8)
Income Tax Expense (Benefit)		(17.8)	(21.4)	(61.2)	(70.2)	(101.6)	(123.5)
Gas Domestic Regulated Revenue Transmission		42.9	46.5	85.9	93.3	180.5	187.4
Questar Pipeline [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period				(22.3)		(22.5)
Income taxes
Total income taxes		0.4	0.0	0.3	(0.1)	0.2	(0.1)
Net other comprehensive income (loss)		(0.4)	(0.1)	(0.5)	0.0	(0.3)	0.2
AOCI at end of period		(22.8)	(22.5)	(22.8)	(22.5)	(22.8)	(22.5)
Income Tax Expense (Benefit)		(7.7)	(8.8)	(16.0)	(16.8)	(32.6)	(34.2)
Gas Domestic Regulated Revenue Transmission		42.9	46.5	85.9	93.3	180.5	187.4
Pension and OPB
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		(160.6)	(175.2)	(163.1)	(178.9)	(171.4)
OCI before reclassifications		0.0	0.0	0.0	0.0
Reclassified from AOCI	[1]	4.0	6.0	8.0	12.0
Income taxes
Attributable to OCI before reclassifications		0.0	0.0	0.0	0.0
Reclassified from AOCI	[2]	(1.5)	(2.2)	(3.0)	(4.5)
Total income taxes		(1.5)	(2.2)	(3.0)	(4.5)
Net other comprehensive income (loss)		2.5	3.8	5.0	7.5
AOCI at end of period		(158.1)	(171.4)	(158.1)	(171.4)	(158.1)	(171.4)
Interest rate cash flow hedges | Questar Pipeline [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		(22.1)	(22.4)	(22.2)	(22.5)	(22.3)
OCI before reclassifications		0.0	0.0	0.0	0.0
Reclassified from AOCI	[1]	0.1	0.2	0.3	0.4
Income taxes
Attributable to OCI before reclassifications		0.0	0.0	0.0	0.0
Reclassified from AOCI	[2]	0.0	(0.1)	(0.1)	(0.2)
Total income taxes		0.0	(0.1)	(0.1)	(0.2)
Net other comprehensive income (loss)		0.1	0.1	0.2	0.2
AOCI at end of period		(22.0)	(22.3)	(22.0)	(22.3)	(22.0)	(22.3)
Commodity cash flow hedges | Questar Pipeline [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		(0.3)	0.0	(0.1)	0.0	(0.2)
OCI before reclassifications		(0.9)	(0.3)	(1.1)	(0.3)
Reclassified from AOCI	[1]	0.0	0.0	0.0	0.0
Income taxes
Attributable to OCI before reclassifications		0.4	0.1	0.4	0.1
Reclassified from AOCI	[2]	0.0	0.0	0.0	0.0
Total income taxes		0.4	0.1	0.4	0.1
Net other comprehensive income (loss)		(0.5)	(0.2)	(0.7)	(0.2)
AOCI at end of period		(0.8)	(0.2)	(0.8)	(0.2)	(0.8)	(0.2)
Long-term investment
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	0.0	0.0
Reclassified from AOCI	[1]	0.0	0.0	0.0	0.0
Income taxes
Attributable to OCI before reclassifications		0.0	0.0	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
Reclassification out of Accumulated Other Comprehensive Income | Interest rate cash flow hedges | Questar Pipeline [Member]
Income taxes
Interest Income (Expense), Net		(0.1)	(0.2)	(0.3)	(0.4)
Income Tax Expense (Benefit)		0.0	0.1	0.1	0.2
Reclassification out of Accumulated Other Comprehensive Income | Commodity cash flow hedges | Questar Pipeline [Member]
Income taxes
Income Tax Expense (Benefit)		0.0	0.0	0.0	0.0
Gas Domestic Regulated Revenue Transmission		$ 0.0	$ 0.0	$ 0.0	$ 0.0

[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.