Regulatory Matters (Regulatory Assets) (Details) - USD ($)
$ in Millions |
Dec. 31, 2015 |
Dec. 31, 2014 |
| Regulatory Assets [Line Items] |
|
|
| Total Regulatory Assets |
$ 139.1
|
|
| Unrecognized Pension Benefit And Other Postretirement Benefit Costs |
|
|
| Regulatory Assets [Line Items] |
|
|
| Total Regulatory Assets |
127.1
|
|
| Other Postretirement Costs |
|
|
| Regulatory Assets [Line Items] |
|
|
| Total Regulatory Assets |
8.9
|
|
| Deferred Taxes on AFUDC Equity |
|
|
| Regulatory Assets [Line Items] |
|
|
| Total Regulatory Assets |
0.0
|
|
| Other Assets |
|
|
| Regulatory Assets [Line Items] |
|
|
| Total Regulatory Assets |
$ 3.1
|
|
| Predecessor |
|
|
| Regulatory Assets [Line Items] |
|
|
| Total Regulatory Assets |
|
$ 158.0
|
| Predecessor | Unrecognized Pension Benefit And Other Postretirement Benefit Costs |
|
|
| Regulatory Assets [Line Items] |
|
|
| Total Regulatory Assets |
|
120.9
|
| Predecessor | Other Postretirement Costs |
|
|
| Regulatory Assets [Line Items] |
|
|
| Total Regulatory Assets |
|
10.8
|
| Predecessor | Deferred Taxes on AFUDC Equity |
|
|
| Regulatory Assets [Line Items] |
|
|
| Total Regulatory Assets |
|
21.8
|
| Predecessor | Other Assets |
|
|
| Regulatory Assets [Line Items] |
|
|
| Total Regulatory Assets |
|
$ 4.5
|