Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Fair Value Disclosures [Line Items]
Gross transfers into Level 3	$ 2.7
Gross transfers out of Level 3	0.2	$ 10.4
Debt Securities | U.S. corporate bonds
Fair Value Disclosures [Line Items]
Gross transfers into Level 3	1.7
Gross transfers out of Level 3		6.8
Debt Securities | Foreign corporate bonds
Fair Value Disclosures [Line Items]
Gross transfers out of Level 3		$ 3.6
Common Stock
Fair Value Disclosures [Line Items]
Gross transfers into Level 3	$ 1.0