Segment Reporting - Adjustments for Performance Fees, Performance Fee Related Compensation and Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Investment income (loss)	$ 584.1	$ 324.6	$ 72.4	$ 1,565.1
Performance revenues related compensation expense	250.6	92.6	343.7	391.6
Net performance revenues	226.8	19.8	387.9	317.5
Adjustments
Segment Reporting Information [Line Items]
Performance revenues related compensation expense	(203.7)	(32.0)	(92.1)	(267.7)
Net performance revenues	(186.9)	37.9	(228.9)	(232.0)
Total Reportable Segments
Segment Reporting Information [Line Items]
Performance revenues related compensation expense	46.9	60.6	251.6	123.9
Net performance revenues	39.9	57.7	159.0	85.5
Performance allocations
Segment Reporting Information [Line Items]
Investment income (loss)	477.4	112.4	731.6	709.1
Performance allocations | Adjustments
Segment Reporting Information [Line Items]
Investment income (loss)	(390.6)	5.9	(321.0)	(499.7)
Performance allocations | Total Reportable Segments
Segment Reporting Information [Line Items]
Investment income (loss)	86.8	118.3	410.6	209.4
Principal investment income (loss)
Segment Reporting Information [Line Items]
Investment income (loss)	106.7	212.2	(659.2)	856.0
Principal investment income (loss) | Adjustments
Segment Reporting Information [Line Items]
Investment income (loss)	(93.9)	(204.9)	710.0	(770.7)
Principal investment income (loss) | Total Reportable Segments
Segment Reporting Information [Line Items]
Investment income (loss)	$ 12.8	$ 7.3	$ 50.8	$ 85.3