Accumulated Other Comprehensive Income (Tables)
 3 Months Ended
Mar. 31, 2015
Statement of Comprehensive Income [Abstract]  
Schedule of Accumulated Other Comprehensive Income (Loss)
Details of the changes in the components of consolidated accumulated other comprehensive income (loss) (AOCI), net of income taxes, as reported in Questar's Condensed Consolidated Balance Sheets, are shown in the tables below. The tables also disclose details of income taxes related to each component of OCI.

 
Pension and OPB
 
Interest rate cash flow hedges
 
Long-term invest.
 
Total
 
Pension and OPB
 
Interest rate cash flow hedges
 
Long-term invest.
 
Total
 
3 Months Ended March 31,
 
3 Months Ended March 31,
 
March 31, 2015
 
March 31, 2014
 
(in millions)
AOCI at beginning of period
$
(178.9
)
 
$
(22.5
)
 
$
0.1

 
$
(201.3
)
 
$
(119.0
)
 
$
(22.9
)
 
$
0.1

 
$
(141.8
)
Reclassified from AOCI(1)
6.0

 
0.2

 
—

 
6.2

 
4.3

 
0.2

 
—

 
4.5

Income taxes
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Reclassified from AOCI(2)
(2.3
)
 
(0.1
)
 
—

 
(2.4
)
 
(1.7
)
 
(0.1
)
 
—

 
(1.8
)
Net other comprehensive income
3.7

 
0.1

 
—

 
3.8

 
2.6

 
0.1

 
—

 
2.7

AOCI at end of period
$
(175.2
)
 
$
(22.4
)
 
$
0.1

 
$
(197.5
)
 
$
(116.4
)
 
$
(22.8
)
 
$
0.1

 
$
(139.1
)

 
Pension and OPB
 
Interest rate cash flow hedges
 
Long-term invest.
 
Total
 
Pension and OPB
 
Interest rate cash flow hedges
 
Long-term invest.
 
Total
 
12 Months Ended
 
12 Months Ended
 
March 31, 2015
 
March 31, 2014
 
(in millions)
AOCI at beginning of period
$
(116.4
)
 
$
(22.8
)
 
$
0.1

 
$
(139.1
)
 
$
(218.3
)
 
$
(23.1
)
 
$
0.1

 
$
(241.3
)
OCI before reclassifications
(113.6
)
 
—

 
—

 
(113.6
)
 
135.8

 
—

 
(0.1
)
 
135.7

Reclassified from AOCI(1)
18.4

 
0.5

 
—

 
18.9

 
29.5

 
0.5

 
—

 
30.0

Income taxes
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
OCI before reclassifications
43.4

 
—

 
—

 
43.4

 
(51.9
)
 
—

 
0.1

 
(51.8
)
Reclassified from AOCI(2)
(7.0
)
 
(0.1
)
 
—

 
(7.1
)
 
(11.5
)
 
(0.2
)
 
—

 
(11.7
)
Total income taxes
36.4

 
(0.1
)
 
—

 
36.3

 
(63.4
)
 
(0.2
)
 
0.1

 
(63.5
)
Net other comprehensive income (loss)
(58.8
)
 
0.4

 
—

 
(58.4
)
 
101.9

 
0.3

 
—

 
102.2

AOCI at end of period
$
(175.2
)
 
$
(22.4
)
 
$
0.1

 
$
(197.5
)
 
$
(116.4
)
 
$
(22.8
)
 
$
0.1

 
$
(139.1
)

(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.