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Commitments and Contingent Liabilities (Black Lung Benefit Liabilities) (Details) $ in Millions |  | 12 Months Ended
--- | --- | ---
Feb. 21, 2020 USD ($) | Dec. 31, 2020 USD ($) claim | Dec. 31, 2019 USD ($) claim | Dec. 31, 2018 USD ($) | Dec. 31, 2013 USD ($)
Black Lung Benefit |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Discount rate |  | 2.00% | 2.90% |  |
Active claims | claim |  | 309 | 324 |  |
Black lung liability |  | $ 64.6 | $ 55.1 |  |
Increase in black lung expense |  | 1.6 |  |  |
Current portion of black lung liability |  | 4.6 | 4.6 |  |
Payments |  | 6.0 | 5.2 | $ 6.3 |
Expense (1) |  | $ 15.4 | $ 10.9 | $ 5.4 |
Uninsured Risk |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Payments | $ 40.4 |  |  |  | $ 8.4

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