|
Fair Value Measurements (Details) (USD $)
In Millions, unless otherwise specified
|
Mar. 31, 2015
|
Dec. 31, 2014
|
Mar. 31, 2014
|
| Carrying Amount [Member] |
|
|
|
| Financial assets [Abstract] |
|
|
|
| Cash and cash equivalents |
$ 4.8 |
$ 32.0 |
$ 5.6 |
| Long-term investment |
16.2 |
15.7 |
15.3 |
| Financial liabilities [Abstract] |
|
|
|
| Short-term debt |
214.0 |
347.0 |
157.0 |
| Long-term debt, including current portion |
1,244.8 |
1,245.2 |
1,246.6 |
| Estimated Fair Value [Member] | Fair Value, Inputs, Level 1 [Member] |
|
|
|
| Financial assets [Abstract] |
|
|
|
| Cash and cash equivalents |
4.8 |
32.0 |
5.6 |
| Financial liabilities [Abstract] |
|
|
|
| Short-term debt |
214.0 |
347.0 |
157.0 |
| Estimated Fair Value [Member] | Fair Value, Inputs, Level 2 [Member] |
|
|
|
| Financial liabilities [Abstract] |
|
|
|
| Long-term debt, including current portion |
1,396.3 |
1,356.1 |
1,357.5 |
| Estimated Fair Value [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] |
|
|
|
| Financial assets [Abstract] |
|
|
|
| Long-term investment |
16.2 |
15.7 |
15.3 |
| Questar Gas [Member] | Carrying Amount [Member] |
|
|
|
| Financial assets [Abstract] |
|
|
|
| Cash and cash equivalents |
0 |
19.8 |
1.4 |
| Notes receivable from Questar |
0 |
0 |
30.6 |
| Financial liabilities [Abstract] |
|
|
|
| Checks outstanding in excess of cash balances |
0.4 |
0 |
0 |
| Notes payable to Questar |
35.8 |
119.3 |
0 |
| Long-term debt, including current portion |
534.5 |
534.5 |
534.5 |
| Questar Gas [Member] | Estimated Fair Value [Member] | Fair Value, Inputs, Level 1 [Member] |
|
|
|
| Financial assets [Abstract] |
|
|
|
| Cash and cash equivalents |
0 |
19.8 |
1.4 |
| Notes receivable from Questar |
0 |
0 |
30.6 |
| Financial liabilities [Abstract] |
|
|
|
| Checks outstanding in excess of cash balances |
0.4 |
0 |
0 |
| Notes payable to Questar |
35.8 |
119.3 |
0 |
| Questar Gas [Member] | Estimated Fair Value [Member] | Fair Value, Inputs, Level 2 [Member] |
|
|
|
| Financial liabilities [Abstract] |
|
|
|
| Long-term debt, including current portion |
629.0 |
607.2 |
588.9 |
| Questar Pipeline [Member] | Carrying Amount [Member] |
|
|
|
| Financial assets [Abstract] |
|
|
|
| Cash and cash equivalents |
6.1 |
7.4 |
4.9 |
| Notes receivable from Questar |
35.9 |
40.1 |
46.6 |
| Financial liabilities [Abstract] |
|
|
|
| Long-term debt, including current portion |
458.8 |
458.8 |
458.8 |
| Questar Pipeline [Member] | Estimated Fair Value [Member] | Fair Value, Inputs, Level 1 [Member] |
|
|
|
| Financial assets [Abstract] |
|
|
|
| Cash and cash equivalents |
6.1 |
7.4 |
4.9 |
| Notes receivable from Questar |
35.9 |
40.1 |
46.6 |
| Questar Pipeline [Member] | Estimated Fair Value [Member] | Fair Value, Inputs, Level 2 [Member] |
|
|
|
| Financial liabilities [Abstract] |
|
|
|
| Long-term debt, including current portion |
$ 515.1 |
$ 495.5 |
$ 509.4 |