Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Net Investment Income [Line Items]
Period of time for which management fees will be received by partners from the initial closing date			10 years
Subsequent period in which management fees are recognized after these fees are called			6 months
Subordinated management fees recognized	$ 7.0
Management fee receivable	97.9		$ 97.9		$ 88.8
Transaction and advisory fees	3.9	$ 9.9	$ 25.5	$ 33.0
Incentive Fees
Performance fees allocation percentage to partnership			20.00%
Percentage of accrued performance fees related to certain Corporate Private Equity funds			25.00%
Interest Income
Interest income from investments	$ 50.8	$ 49.8	$ 155.0	$ 144.5
Minimum
Net Investment Income [Line Items]
Percentage of management fees earned			1.00%
Percentage of management fees earned on unrealized investments after termination of the investment period			0.60%
Extension period for fund closing			1 year
Percentage of management fees for CLOs on the total par amount of assets in the fund			0.40%
Period of management fees related to collateralized loan obligation			5 years
Percentage of management fees for business development companies			1.25%
Percentage of management fees from fund of funds following the expiration of the weighted-average investment period			0.25%
Incentive Fees
External coinvestment vehicles percentage			10.00%
Percentage of allocation based performance fees related to fund of funds vehicles			2.00%
Percent of preferred returns			7.00%
Minimum | Managed Accounts and Longer-dated Carry Funds
Net Investment Income [Line Items]
Percentage of management fees earned			0.20%
Minimum | Private Equity And Real Estate Fund
Net Investment Income [Line Items]
Percentage of management fees from funds of funds during the commitment fee period			0.25%
Minimum | Certain Open-ended and Longer-dated Carry Funds
Incentive Fees
Percent of preferred returns			4.00%
Maximum
Net Investment Income [Line Items]
Percentage of management fees earned			2.00%
Percentage of management fees earned on unrealized investments after termination of the investment period			2.00%
Extension period for fund closing			2 years
Percentage of management fees for CLOs on the total par amount of assets in the fund			0.50%
Period of management fees related to collateralized loan obligation			10 years
Percentage of management fees for business development companies			1.50%
Percentage of management fees from fund of funds following the expiration of the weighted-average investment period			1.00%
Incentive Fees
External coinvestment vehicles percentage			20.00%
Percentage of allocation based performance fees related to fund of funds vehicles			10.00%
Percent of preferred returns			9.00%
Maximum | Managed Accounts and Longer-dated Carry Funds
Net Investment Income [Line Items]
Percentage of management fees earned			1.00%
Maximum | Private Equity And Real Estate Fund
Net Investment Income [Line Items]
Percentage of management fees from funds of funds during the commitment fee period			1.00%
Maximum | Certain Open-ended and Longer-dated Carry Funds
Incentive Fees
Percent of preferred returns			7.00%