Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3		$ 5.8		$ 16.2	$ 238.1
Gross transfers into Level 3		$ 9.6		9.6	38.1
Gain (loss) on conversion of private equity interests into limited partnership interests			$ 0.0
Debt Securities | U.S. corporate bonds
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3				10.7
Gross transfers into Level 3				3.9	9.0
Debt Securities | Foreign corporate bonds
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3				5.5
Gross transfers into Level 3				0.7
Other invested assets
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3	[1]				$ 238.1
Gross transfers into Level 3	[1]			$ 5.0
Other invested assets | Ares
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3			$ 232.9

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