Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended			12 Months Ended
		Mar. 31, 2016		Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		$ (185.3)		$ (201.3)	$ (197.5)
OCI before reclassifications		(0.2)
Reclassified from AOCI	[2]	4.2	[1]	6.2
Income taxes
Reclassified from AOCI	[4]	(1.6)	[3]	(2.4)
Total income taxes		(1.6)		(2.4)	(9.2)	$ 36.3
Net other comprehensive income (loss)		2.4		3.8	14.6	(58.4)
AOCI at end of period		(182.9)		(197.5)	(182.9)	(197.5)
Pension and OPB
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		(163.1)		(178.9)	(175.2)
OCI before reclassifications		0.0
Reclassified from AOCI	[2]	4.0	[1]	6.0
Income taxes
Reclassified from AOCI	[4]	(1.5)	[3]	(2.3)
Total income taxes		(1.5)		(2.3)
Net other comprehensive income (loss)		2.5		3.7
AOCI at end of period		(160.6)		(175.2)	(160.6)	(175.2)
Interest rate cash flow hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		(22.2)		(22.5)	(22.4)
OCI before reclassifications		0.0
Reclassified from AOCI	[2]	0.2	[1]	0.2
Income taxes
Reclassified from AOCI	[4]	(0.1)	[3]	(0.1)
Total income taxes		(0.1)		(0.1)
Net other comprehensive income (loss)		0.1		0.1
AOCI at end of period		(22.1)		(22.4)	(22.1)	(22.4)
Commodity cash flow hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		(0.1)
OCI before reclassifications		(0.2)
Reclassified from AOCI	[1],[2]	0.0
Income taxes
Reclassified from AOCI	[3],[4]	0.0
Total income taxes		0.0
Net other comprehensive income (loss)		(0.2)
AOCI at end of period		(0.3)			(0.3)
Long-term investment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
AOCI at beginning of period		0.1		0.1	0.1
OCI before reclassifications		0.0
Reclassified from AOCI	[2]	0.0	[1]	0.0
Income taxes
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
AOCI at end of period		$ 0.1		$ 0.1	$ 0.1	$ 0.1

[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.