Derivative Financial Instruments - Derivatives Not Designated as Hedging Instruments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Effect of derivatives not designated as hedging instruments on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	$ 2.8	$ (33.7)	$ 5.7	$ (24.3)
Interest rate contracts
Effect of derivatives not designated as hedging instruments on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	94.1	(19.8)	161.4	(67.8)
Foreign exchange contracts
Effect of derivatives not designated as hedging instruments on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	5.4	(24.2)	10.0	(15.4)
Equity contracts
Effect of derivatives not designated as hedging instruments on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	(15.8)	(43.0)	(90.8)	(58.2)
Credit contracts
Effect of derivatives not designated as hedging instruments on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	(4.8)	1.2	(3.5)	1.3
Other contracts
Effect of derivatives not designated as hedging instruments on the consolidated statements of operations
Amount of gain (loss) recognized in net income on derivatives	$ (76.1)	$ 52.1	$ (71.4)	$ 115.8