Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)		3 Months Ended			9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3					$ 19,300,000	$ 238,100,000
Gross transfers into Level 3		$ 5,900,000			15,500,000	44,400,000
Gain (loss) on conversion of private equity interests into limited partnership interests				$ 0
Debt Securities
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3		$ 3,000,000			19,300,000
Gross transfers into Level 3			$ 6,300,000			44,400,000
Debt Securities | U.S. corporate bonds
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3					13,800,000
Gross transfers into Level 3					9,800,000	15,000,000
Debt Securities | Foreign corporate bonds
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3					5,500,000
Gross transfers into Level 3					700,000	300,000
Debt Securities | Mortgage and asset backed securities | Other asset-backed securities
Fair Value Disclosures [Line Items]
Gross transfers into Level 3						29,100,000
Debt Securities | Mortgage and asset backed securities | Other asset-backed securities | Collateralized loan obligations
Fair Value Disclosures [Line Items]
Gross transfers into Level 3						29,100,000
Other invested assets
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3	[1]					$ 238,100,000
Gross transfers into Level 3	[1]				$ 5,000,000
Other invested assets | Ares
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3				$ 232,900,000

[1] Includes partnership and non-marketable equity investments accounted for on an AFS basis.