Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accounting Policies [Abstract]
Interest income from investments	$ 57.9	$ 41.8	$ 155.9	$ 124.1
Accrued performance allocations and incentive fee-related compensation	2,083.8		$ 2,083.8		$ 1,894.8
Percentage of estimated realizable tax benefit to be paid by corporate taxpayers on exchange transactions			85.00%
Percentage of remaining estimated realizable tax benefit on exchange transactions			15.00%
Foreign currency transaction (gain) loss	$ 3.2	$ 0.7	$ 1.6	$ 2.2