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Amount receivable from broker-dealers and clearing organizations, including, but not limited to, securities failed-to-deliver, certain deposits for securities borrowed, open transactions, good faith and margin deposits, commissions and floor brokerage receivables.

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Derivatives and Fair Value Measurements - Textuals (Details) T in Millions, $ in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) T	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) T	Jun. 30, 2019 USD ($)	Dec. 31, 2020 T $ / $	Jun. 30, 2020 AUD ($) T	Dec. 31, 2019 USD ($)
Derivatives, Fair Value [Line Items]
Transfers between fair value levels 1, 2, and 3	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount						$ 613.0
Coal
Derivatives, Fair Value [Line Items]
Derivative, Nonmonetary Notional Amount \| T	1.2		1.2			1.2
Diesel Fuel Hedge Contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount						$ 0.0
Scenario, Forecast \| Coal
Derivatives, Fair Value [Line Items]
Derivative, Nonmonetary Notional Amount \| T					0.9
Scenario, Forecast \| Minimum \| Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, exchange rate cap \| $ / $					0.70
Scenario, Forecast \| Maximum \| Foreign currency option contracts
Derivatives, Fair Value [Line Items]
Derivative, exchange rate cap \| $ / $					0.75
Coal Trading
Derivatives, Fair Value [Line Items]
Derivative, initial margin posted	$ 4.5		$ 4.5				$ 7.9
Derivatives, amount posted in excess of initial margin requirements	1.7		1.7				1.3
Derivative, variation margin	$ 23.8		$ 23.8				$ 22.1