Employee Benefits (Tables)

12 Months Ended

Dec. 31, 2014

Compensation and Retirement Disclosure [Abstract]

Schedule of pension and other postretirement benefit plan asset allocation

Qualified pension and other postretirement benefit plan assets were invested as follows:

	Actual Allocation				Policy Range
	December 31,				December 31,
	2014		2013		2014					2013
Total domestic equity securities	38	%	38	%	35	%	-	45	%	35	%	-	45	%
Foreign equity securities
Developed market foreign equity securities	18	%	20	%
Emerging market foreign equity securities	4	%	5	%
Total foreign securities	22	%	25	%	25	%	-	35	%	25	%	-	35	%
Debt securities
Investment-grade intermediate-term debt	7	%	7	%
Investment-grade long-term debt	14	%	11	%
Below-investment-grade debt	9	%	9	%
Total debt securities	30	%	27	%	25	%	-	35	%	25	%	-	35	%
Inflation protection securities	9	%	9	%	—	%	-	10	%	—	%	-	10	%
Cash and short-term investments	1	%	1	%	—	%	-	3	%	—	%	-	3	%

Pension and postretirement benefit assets fair value by level

The following tables set forth, by level within the fair value hierarchy, qualified pension and other postretirement benefit plan assets at fair value as of December 31, 2014 and 2013:

	December 31, 2014
	Level 1		Level 2		Total
	(in millions)
Corporate bonds	$	—	$	1.5	$	1.5
U.S. government securities	—		12.9		12.9
Registered investment companies:
Fixed income funds	114.9		—		114.9
Inflation protection fund	34.5		—		34.5
Domestic equity fund	1.6		—		1.6
Commingled funds:
Cash equivalent funds	—		6.7		6.7
Domestic equity index fund	—		275.8		275.8
Foreign equity growth fund	—		41.2		41.2
Foreign equity index funds	—		77.8		77.8
Corporate debt funds	—		91.0		91.0
Inflation protection fund	—		33.1		33.1
Foreign equity growth 103-12 investment entity	—		46.2		46.2
Total	$	151.0	$	586.2	$	737.2

	December 31, 2013
	Level 1		Level 2		Total
	(in millions)
Corporate bonds	$	—	$	1.7	$	1.7
U.S. government securities	—		1.6		1.6
Registered investment companies:
Fixed income funds	110.7		—		110.7
Inflation protection fund	29.4		—		29.4
Domestic equity fund	2.0		—		2.0
Commingled funds:
Cash equivalent funds	—		3.5		3.5
Domestic equity index fund	—		256.2		256.2
Foreign equity growth fund	—		42.9		42.9
Foreign equity index funds	—		76.0		76.0
Corporate debt funds	—		72.1		72.1
Inflation protection fund	—		28.8		28.8
Foreign equity growth 103-12 investment entity	—		47.1		47.1
Total	$	142.1	$	529.9	$	672.0

Assumptions used to calculate benefit obligations

The qualified pension projected benefit obligation was measured using the following assumptions at December 31:

	2014		2013
Discount rate	4.10	%	4.90	%
Rate of increase in compensation	5.50		5.50
Long-term return on assets	7.00		7.25

The nonqualified pension projected benefit obligation was measured using the following assumptions at December 31:

	2014		2013
Discount rate	2.90	%	3.30	%
Rate of increase in compensation	5.50		5.50

The postretirement accumulated benefit obligation was measured using the following assumptions at December 31:

	2014					2013
Discount rate	4.00				%	4.70				%
Long-term return on assets	7.00					7.25
Health-care inflation rate	7.50					8.00
	decreasing to					decreasing to
	4.50	%	by	2021		4.50	%	by	2021

Changes in benefit obligation, changes in fair value of plan assets, and underfunded status

Plan obligations and fair value of all plan assets are shown in the following table:

	Pension						Other Postretirement Benefits
	Year Ended December 31,						Year Ended December 31,
	2014			2013			2014			2013
	(in millions)
Change in benefit obligation
Benefit obligation at beginning of year	$	702.7		$	760.3		$	86.3		$	94.4
Service cost	11.8			13.9			0.6			0.7
Interest cost	33.4			30.6			3.8			3.7
Change in plan assumptions	109.1			(73.6		)	4.9			(8.6		)
Actuarial (gain)	(0.7		)	(7.7		)	(3.0		)	(0.5		)
Benefits paid	(22.3		)	(20.8		)	(1.8		)	(3.4		)
Benefit obligation at end of year	834.0			702.7			90.8			86.3
Change in plan assets
Fair value of plan assets at beginning of year	628.2			510.8			43.8			37.6
Actual gain on plan assets	40.4			80.2			2.9			5.8
Company contributions to the plan	43.9			58.0			2.1			3.8
Benefits paid	(22.3		)	(20.8		)	(1.8		)	(3.4		)
Fair value of plan assets at end of year	690.2			628.2			47.0			43.8
Underfunded status (current and long-term)	$	(143.8	)	$	(74.5	)	$	(43.8	)	$	(42.5	)

Estimated benefit plan payments for the next five years and the subsequent five years aggregated

Estimated benefit plan payments for the five years following 2014 and the subsequent five years aggregated are as follows:

	Pension		Other Postretirement Benefits
	Years Ending December 31,
	(in millions)
2015	$	67.3	$	4.3
2016	52.6		4.5
2017	47.2		4.6
2018	63.0		4.8
2019	53.6		5.0
2020 through 2024	265.0		26.3

Net pension and other postretirement benefit cost components

The components of the net pension and other postretirement benefit costs are as follows. The net pension cost includes both the qualified and nonqualified pension plans.

	Pension									Other Postretirement Benefits
	Year Ended December 31,									Year Ended December 31,
	2014			2013			2012			2014			2013			2012
	(in millions)
Service cost	$	11.8		$	13.9		$	13.2		$	0.6		$	0.7		$	0.7
Interest cost	33.4			30.6			31.1			3.8			3.7			3.8
Expected return on plan assets	(43.6		)	(38.1		)	(31.8		)	(3.0		)	(2.6		)	(2.3		)
Prior service and other costs	0.6			1.1			1.1			—			—			1.6
Recognized net actuarial loss	15.4			28.9			24.9			0.7			3.0			2.5
Accretion of regulatory liability	—			—			—			0.8			0.5			0.3
Net periodic cost	$	17.6		$	36.4		$	38.5		$	2.9		$	5.3		$	6.6

Assumptions used to calculate net benefit costs

Assumptions at January 1, used to calculate the qualified net pension cost for the years, were as follows:

	2014		2013		2012
Discount rate	4.90	%	4.20	%	4.80	%
Rate of increase in compensation	5.50		5.50		5.50
Long-term return on assets	7.25		7.25		7.25

Assumptions at January 1, used to calculate the nonqualified net pension cost for the years, were as follows:

	2014		2013		2012
Discount rate	3.30	%	2.40	%	3.30	%
Rate of increase in compensation	5.50		5.50		5.50

Assumptions at January 1, used to calculate the net postretirement benefit cost for the years, were as follows:

	2014					2013					2012
Discount rate	4.70				%	4.00				%	4.40				%
Long-term return on assets	7.25					7.25					7.25
Health-care inflation rate	8.00					8.50					7.00
	decreasing to					decreasing to					decreasing to
	4.50	%	by	2021		4.50	%	by	2021		5.00	%	by	2014

Subsidiary net cost and contribution information

Pension and other postretirement benefit net cost and plan contribution information for Questar Gas and Questar Pipeline are shown below:

	Pension						Other Postretirement Benefits
	Year Ended December 31,						Year Ended December 31,
	2014		2013		2012		2014			2013		2012
	(in millions)
Questar Gas
Net periodic cost	$	8.5	$	18.1	$	19.7	$	0.8		$	2.4	$	3.3
Share of total plan contributions	21.8		29.6		39.4		1.1			2.0		1.9
Questar Pipeline
Net periodic cost	$	3.2	$	6.7	$	7.4	$	(0.4	)	$	0.2	$	1.2
Share of total plan contributions	8.1		11.2		15.1		0.4			0.7		0.6