Fair Value Disclosures (Nonrecurring Fair Value Measurements Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
Investment in FXCM
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity method investment impairment		$ 130.2
Investment in FXCM | Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity method investments, fair value		$ 186.7
Investment In Golden Queen
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity method investment impairment	$ 47.9		$ 47.9
Investment In Golden Queen | Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity method investments, fair value				$ 62.3
Measurement Input, Discount Rate | Investment in FXCM | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity method investment, measurement input		0.15
Measurement Input, Discount Rate | Investment In Golden Queen | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity method investment, measurement input				0.12
Measurement Input, EBITDA Multiple | Investment in FXCM | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity method investment, measurement input		5.4
Measurement Input, Revenue Multiple | Investment in FXCM | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity method investment, measurement input		1.5