Pension and Other Postretirement Benefit Plans - Recurring Fair Value Measures, Activity in Level 3 (Details) - USD ($) $ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2019 |
Dec. 31, 2018 |
|
| Pension [Member] | Private Equity Funds [Member] | ||
| Activity in Level 3 [Roll Forward] | ||
| Beginning Balance | $ 27.8 | $ 33.2 |
| Actual Return on Plan Assets | 0.4 | 2.8 |
| Purchases, Sales, and Settlements – Net | (20.2) | (8.2) |
| Ending Balance | 8.0 | 27.8 |
| Pension [Member] | Real Estate [Member] | ||
| Activity in Level 3 [Roll Forward] | ||
| Beginning Balance | 20.8 | 25.5 |
| Actual Return on Plan Assets | (1.3) | 0.7 |
| Purchases, Sales, and Settlements – Net | (1.5) | (5.4) |
| Ending Balance | 18.0 | 20.8 |
| Postretirement Health and Life [Member] | Private Equity Funds [Member] | ||
| Activity in Level 3 [Roll Forward] | ||
| Beginning Balance | 6.5 | 8.2 |
| Actual Return on Plan Assets | 0.7 | 0.9 |
| Purchases, Sales, and Settlements – Net | (5.5) | (2.6) |
| Ending Balance | $ 1.7 | $ 6.5 |