Fair Value Measurements - Summary of Estimated Fair Values (Details) - USD ($)
$ in Millions |
6 Months Ended |
12 Months Ended |
Jun. 30, 2021 |
Dec. 31, 2020 |
| Mutual Fund Investments, Concentration Risk | Common Stock, Large Cap US Companies | Investment Benchmark |
|
|
| Liability Class: |
|
|
| Concentration risk percentage |
30.00%
|
30.00%
|
| Mutual Fund Investments, Concentration Risk | Common Stock of Small to Mid Cap US Companies | Investment Benchmark |
|
|
| Liability Class: |
|
|
| Concentration risk percentage |
10.00%
|
10.00%
|
| Mutual Fund Investments, Concentration Risk | Common Stock, International Companies | Investment Benchmark |
|
|
| Liability Class: |
|
|
| Concentration risk percentage |
10.00%
|
5.00%
|
| Mutual Fund Investments, Concentration Risk | Bond Funds | Investment Benchmark |
|
|
| Liability Class: |
|
|
| Concentration risk percentage |
15.00%
|
15.00%
|
| Mutual Fund Investments, Concentration Risk | Short-Term Investments | Investment Benchmark |
|
|
| Liability Class: |
|
|
| Concentration risk percentage |
5.00%
|
15.00%
|
| Mutual Fund Investments, Concentration Risk | Blended Funds | Investment Benchmark |
|
|
| Liability Class: |
|
|
| Concentration risk percentage |
30.00%
|
25.00%
|
| Fair Value, Measurements, Recurring |
|
|
| Asset Class: |
|
|
| Mutual fund investments in a non-qualified savings plan |
$ 1.9
|
$ 1.9
|
| Total Assets |
3.7
|
2.1
|
| Liability Class: |
|
|
| Derivative - foreign exchange instrument |
(9.6)
|
|
| Liabilities related to non-qualified savings plan |
(1.9)
|
(1.9)
|
| Total Liabilities |
(11.5)
|
(1.9)
|
| Fair Value, Measurements, Recurring | Natural gas instruments, net |
|
|
| Asset Class: |
|
|
| Derivatives – natural gas instruments, net |
1.8
|
0.2
|
| Fair Value, Measurements, Recurring | Level One |
|
|
| Asset Class: |
|
|
| Mutual fund investments in a non-qualified savings plan |
1.9
|
1.9
|
| Total Assets |
1.9
|
1.9
|
| Liability Class: |
|
|
| Derivative - foreign exchange instrument |
0.0
|
|
| Liabilities related to non-qualified savings plan |
(1.9)
|
(1.9)
|
| Total Liabilities |
(1.9)
|
(1.9)
|
| Fair Value, Measurements, Recurring | Level One | Natural gas instruments, net |
|
|
| Asset Class: |
|
|
| Derivatives – natural gas instruments, net |
0.0
|
0.0
|
| Fair Value, Measurements, Recurring | Level Two |
|
|
| Asset Class: |
|
|
| Mutual fund investments in a non-qualified savings plan |
0.0
|
0.0
|
| Total Assets |
1.8
|
0.2
|
| Liability Class: |
|
|
| Derivative - foreign exchange instrument |
(9.6)
|
|
| Liabilities related to non-qualified savings plan |
0.0
|
0.0
|
| Total Liabilities |
(9.6)
|
0.0
|
| Fair Value, Measurements, Recurring | Level Two | Natural gas instruments, net |
|
|
| Asset Class: |
|
|
| Derivatives – natural gas instruments, net |
1.8
|
0.2
|
| Fair Value, Measurements, Recurring | Level Three |
|
|
| Asset Class: |
|
|
| Mutual fund investments in a non-qualified savings plan |
0.0
|
0.0
|
| Total Assets |
0.0
|
0.0
|
| Liability Class: |
|
|
| Derivative - foreign exchange instrument |
0.0
|
|
| Liabilities related to non-qualified savings plan |
0.0
|
0.0
|
| Total Liabilities |
0.0
|
0.0
|
| Fair Value, Measurements, Recurring | Level Three | Natural gas instruments, net |
|
|
| Asset Class: |
|
|
| Derivatives – natural gas instruments, net |
$ 0.0
|
$ 0.0
|