Fair Value of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3	$ 10.4	$ 5.2
Gross transfers into Level 3	0	35.0
Debt Securities | U.S. corporate bonds
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3	6.8
Gross transfers into Level 3		5.9
Debt Securities | Foreign corporate bonds
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3	3.6
Debt Securities | Mortgage and asset backed securities | Other asset-backed securities
Fair Value Disclosures [Line Items]
Gross transfers into Level 3		29.1
Debt Securities | Mortgage and asset backed securities | Other asset-backed securities | Collateralized loan obligations
Fair Value Disclosures [Line Items]
Gross transfers into Level 3		$ 35.0