Fair Value Measurement - Summary of Quantitative Information about Level III Inputs (Details) $ in Millions	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of assets	$ 6,957.2	$ 6,628.4
Fair value of liabilities	5,938.5	5,563.4
Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of assets	6,876.3	6,627.7
Fair value of liabilities	5,938.3	5,563.0
Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of liabilities	5,700.1	5,358.9
Investments of Consolidated Funds | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of assets	6,452.9	6,056.9
Consensus Pricing | Equity securities | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of assets	18.9	9.4
Consensus Pricing | Bonds | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of assets	581.1	550.4
Consensus Pricing | Loans | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of assets	$ 5,834.3	5,497.1
Consensus Pricing | Loans | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of assets		$ 14.1
Consensus Pricing | Indicative Quotes | Minimum | Equity securities | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.00	0.00
Consensus Pricing | Indicative Quotes | Minimum | Bonds | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	94	0.85
Consensus Pricing | Indicative Quotes | Minimum | Loans | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.37	0.15
Consensus Pricing | Indicative Quotes | Minimum | Loans | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input		0.98
Consensus Pricing | Indicative Quotes | Maximum | Equity securities | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	74.50	40.00
Consensus Pricing | Indicative Quotes | Maximum | Bonds | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	1.09	1.08
Consensus Pricing | Indicative Quotes | Maximum | Loans | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	1.09	1.08
Consensus Pricing | Indicative Quotes | Maximum | Loans | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input		1
Consensus Pricing | Indicative Quotes | Weighted Average | Equity securities | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.66	0.57
Consensus Pricing | Indicative Quotes | Weighted Average | Bonds | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	1	0.98
Consensus Pricing | Indicative Quotes | Weighted Average | Loans | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.99	0.97
Consensus Pricing | Indicative Quotes | Weighted Average | Loans | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input		1
Consensus Pricing with Discounted Cash Flow | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of liabilities	$ 238.2	$ 204.1
Consensus Pricing with Discounted Cash Flow | Loans | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of assets	18.6
Consensus Pricing with Discounted Cash Flow | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of assets	274.9	437.0
Consensus Pricing with Discounted Cash Flow | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of assets	$ 69.8	$ 52.5
Consensus Pricing with Discounted Cash Flow | Indicative Quotes | Minimum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Liabilities, measurement input	0.42	0.30
Consensus Pricing with Discounted Cash Flow | Indicative Quotes | Minimum | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.84	0.71
Consensus Pricing with Discounted Cash Flow | Indicative Quotes | Minimum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.43	0.31
Consensus Pricing with Discounted Cash Flow | Indicative Quotes | Maximum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Liabilities, measurement input	0.77	0.91
Consensus Pricing with Discounted Cash Flow | Indicative Quotes | Maximum | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	1.01	1
Consensus Pricing with Discounted Cash Flow | Indicative Quotes | Maximum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.88	0.90
Consensus Pricing with Discounted Cash Flow | Indicative Quotes | Weighted Average | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Liabilities, measurement input	0.56	0.50
Consensus Pricing with Discounted Cash Flow | Indicative Quotes | Weighted Average | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.99	0.98
Consensus Pricing with Discounted Cash Flow | Indicative Quotes | Weighted Average | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.72	0.46
Consensus Pricing with Discounted Cash Flow | Discount Margins | Minimum | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.0050	0.0085
Consensus Pricing with Discounted Cash Flow | Discount Margins | Maximum | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.1330	0.1725
Consensus Pricing with Discounted Cash Flow | Discount Margins | Weighted Average | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.0256	0.0227
Consensus Pricing with Discounted Cash Flow | Default Rates | Minimum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Liabilities, measurement input	0.01	0.01
Consensus Pricing with Discounted Cash Flow | Default Rates | Minimum | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.01	0.01
Consensus Pricing with Discounted Cash Flow | Default Rates | Minimum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.01	0.01
Consensus Pricing with Discounted Cash Flow | Default Rates | Maximum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Liabilities, measurement input	0.02	0.02
Consensus Pricing with Discounted Cash Flow | Default Rates | Maximum | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.02	0.02
Consensus Pricing with Discounted Cash Flow | Default Rates | Maximum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.02	0.02
Consensus Pricing with Discounted Cash Flow | Default Rates | Weighted Average | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Liabilities, measurement input	0.01	0.01
Consensus Pricing with Discounted Cash Flow | Default Rates | Weighted Average | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.01	0.01
Consensus Pricing with Discounted Cash Flow | Default Rates | Weighted Average | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.01	0.01
Consensus Pricing with Discounted Cash Flow | Recovery Rates | Minimum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Liabilities, measurement input	0.50	0.50
Consensus Pricing with Discounted Cash Flow | Recovery Rates | Minimum | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.50	0.50
Consensus Pricing with Discounted Cash Flow | Recovery Rates | Minimum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.50	0.50
Consensus Pricing with Discounted Cash Flow | Recovery Rates | Maximum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Liabilities, measurement input	0.70	0.70
Consensus Pricing with Discounted Cash Flow | Recovery Rates | Maximum | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.70	0.70
Consensus Pricing with Discounted Cash Flow | Recovery Rates | Maximum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.70	0.70
Consensus Pricing with Discounted Cash Flow | Recovery Rates | Weighted Average | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Liabilities, measurement input	0.60	0.60
Consensus Pricing with Discounted Cash Flow | Recovery Rates | Weighted Average | Senior secured notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.60	0.60
Consensus Pricing with Discounted Cash Flow | Recovery Rates | Weighted Average | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.60	0.60
Consensus Pricing with Discounted Cash Flow | Discount Rates | Minimum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Liabilities, measurement input	0.15	0.16
Consensus Pricing with Discounted Cash Flow | Discount Rates | Minimum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.10	0.16
Consensus Pricing with Discounted Cash Flow | Discount Rates | Maximum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Liabilities, measurement input	0.25	0.30
Consensus Pricing with Discounted Cash Flow | Discount Rates | Maximum | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.25	0.30
Consensus Pricing with Discounted Cash Flow | Discount Rates | Weighted Average | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Liabilities, measurement input	0.20	0.22
Consensus Pricing with Discounted Cash Flow | Discount Rates | Weighted Average | Subordinated notes and preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.17	0.23
Discounted Cash Flow | BDC preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of assets	$ 71.8	$ 60.0
Discounted Cash Flow | Aviation subordinated notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of assets	$ 6.9	$ 7.2
Discounted Cash Flow | Discount Rates | Minimum | Loans | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.03
Discounted Cash Flow | Discount Rates | Minimum | BDC preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.07	0.07
Discounted Cash Flow | Discount Rates | Minimum | Aviation subordinated notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.16	0.20
Discounted Cash Flow | Discount Rates | Maximum | Loans | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.08
Discounted Cash Flow | Discount Rates | Maximum | BDC preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.07	0.07
Discounted Cash Flow | Discount Rates | Maximum | Aviation subordinated notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.16	0.20
Discounted Cash Flow | Discount Rates | Weighted Average | Loans | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.06
Discounted Cash Flow | Discount Rates | Weighted Average | BDC preferred shares | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.07	0.07
Discounted Cash Flow | Discount Rates | Weighted Average | Aviation subordinated notes | Level III
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets, measurement input	0.16	0.20