Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions			3 Months Ended
		Mar. 15, 2018	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3			$ 1.6	$ 4.1
Gross transfers into Level 3			63.0	2.6
Equity Securities | Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3				2.3
Gross transfers into Level 3				0.4
Equity Securities | Common Stock | Ares | Insurance Segment
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 1		$ 208.2
Equity Securities | Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3			3.1
Debt Securities | U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3				1.8
Gross transfers into Level 3			1.2	1.9
Debt Securities | Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3				$ 0.3
Debt Securities | Mortgage and asset backed securities | CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3			1.6
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 | Ares | Insurance Segment
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3		$ 58.7
All Investments Accounted for at Fair Value Other Than Other Invested Assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3			$ 4.3

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