Fair Value Disclosure (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	May 02, 2022	Dec. 31, 2020
Coal Mining
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value gained from contract termination					$ 4.1
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized (loss) gain on equity securities	$ (0.4)	$ 0.6	$ 0.8	$ 1.1
Significant Unobservable Inputs (Level 3) | Fair value measurements, nonrecurring | Coal Mining
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value gained from contract termination					4.1
Investment	5.0		5.0		$ 5.0
Fair value of privately-held company	12.8		12.8
Bellaire | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized (loss) gain on equity securities	$ (1.5)	$ 0.7	$ (2.1)	$ 1.0
Carrying Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities cost						$ 2.0
Carrying Value | Bellaire | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities cost						$ 5.0