Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions				12 Months Ended
		Sep. 24, 2018	Mar. 15, 2018	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3				$ 221.6	$ 77.8
Gross transfers into Level 3				65.0	5.5
All Investment Issuers Other Than Ares
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3				164.7
Gross transfers into Level 3				6.3
Ares
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3		$ 56.9
Securities Other Than Other Asset Backed Securities and U.S. Corporate Bonds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3					20.3
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3				150.6	5.3
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3					3.1
Gross transfers into Level 3				4.3
Securities Other Than Residential Mortgage Backed Securities And Other Asset Backed Securities [Member] | All Investment Issuers Other Than Ares
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3				3.1
Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3					3.2
Gross transfers into Level 3					1.4
Common Stock | Ares | Insurance Segment
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 1		$ 56.9	$ 208.2
Other Asset Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3				11.0	36.2
U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3				1.3	21.3
Gross transfers into Level 3				1.9	3.8
Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3				0.2	2.2
Gross transfers into Level 3					$ 0.3
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3	[1]			56.9
Gross transfers into Level 3	[1]			58.8
Other invested assets | All Investment Issuers Other Than Ares
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3				0.1
Other invested assets | Ares
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfer out of Level 3				$ 56.9
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

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