Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions				3 Months Ended
		Sep. 24, 2018	Mar. 15, 2018	Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3				$ 1.0	$ 1.6
Gross transfers into Level 3				14.7	63.0
All Investment Issuers Other Than Ares
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3					4.3
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3					3.1
Common Stock | Ares
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 1		$ 56.9	$ 208.2
Common Stock | Ares | Insurance Segment
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 1			208.2
U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3				1.0
Gross transfers into Level 3					1.2
Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3				$ 14.7
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3	[1]				58.7
Other invested assets | Limited partnership interests in certain subsidiaries of Ares | Insurance Segment
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3		$ 56.9
Gross transfers into Level 3			$ 58.7		$ 58.7

[1] Includes partnership and non-marketable equity investments accounted for at fair value.