Fair Value - Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($)
$ in Millions |
12 Months Ended |
|
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
| Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] |
|
|
|
| Recurring Fair Value Measure, Unobservable Inputs, Period Increase (Decrease) |
$ 3.8
|
|
|
| Equity Investments |
197.6
|
$ 161.1
|
|
| ATC [Member] |
|
|
|
| Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] |
|
|
|
| Equity Investments |
141.6
|
128.1
|
$ 118.7
|
| Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] | Corporate and Other [Member] |
|
|
|
| Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] |
|
|
|
| Impairment of Long-Lived Assets Held-for-use |
0.0
|
|
|
| Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] | ATC [Member] | Regulated Operations [Member] |
|
|
|
| Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] |
|
|
|
| Equity Method Investment, Indicators of Impairment |
$ 0.0
|
$ 0.0
|
|