|
Basis of Presentation of Interim Financial Statements (Details) (USD $)
In Millions, unless otherwise specified
|
3 Months Ended |
12 Months Ended |
|
Mar. 31, 2015
|
Mar. 31, 2014
|
Mar. 31, 2015
|
Mar. 31, 2014
|
| Cost of Sales [Abstract] |
|
|
|
|
| Other cost of sales |
$ 0.3 |
$ 0.1 |
$ 1.1 |
$ 0.2 |
| Total cost of sales |
131.8 |
148.2 |
169.9 |
234.4 |
| Questar Gas [Member] |
|
|
|
|
| Payments for Questar Gas Acquisition |
11.4 |
|
11.4 |
|
| Additional Customers from Questar Gas Acquisition |
6,500 |
|
|
|
| Cost of Sales [Abstract] |
|
|
|
|
| Gas purchases |
41.0 |
92.7 |
84.8 |
176.0 |
| Operator service fee |
84.8 |
89.1 |
345.4 |
310.9 |
| Transportation and storage |
22.2 |
22.1 |
79.7 |
80.2 |
| Gathering |
5.4 |
4.9 |
21.5 |
19.1 |
| Royalties |
11.9 |
17.3 |
54.7 |
50.5 |
| Storage (injection) withdrawal, net |
30.9 |
33.0 |
(3.2) |
3.4 |
| Purchased-gas account adjustment |
36.2 |
(5.6) |
(4.0) |
(30.3) |
| Other |
1.2 |
1.1 |
4.9 |
4.8 |
| Total Questar Gas cost of natural gas sold |
233.6 |
254.6 |
583.8 |
614.6 |
| Elimination of Questar Gas cost of natural gas sold - affiliated companies |
(104.0) |
(108.2) |
(419.2) |
(387.0) |
| Total Questar Gas cost of natural gas sold - unaffiliated parties |
129.6 |
146.4 |
164.6 |
227.6 |
| Questar Pipeline [Member] |
|
|
|
|
| Ownership percentage in unconsolidated affiliate (in hundredths) |
50.00% |
|
50.00% |
|
| Cost of Sales [Abstract] |
|
|
|
|
| Total Questar Pipeline cost of sales |
$ 1.9 |
$ 1.7 |
$ 4.2 |
$ 6.6 |