Fair Value of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
Fair Value Disclosures [Line Items]
Gross transfers out of Level 2 into Level 3	$ 21.7	$ 21.7	[1]

[1] Principally due to a decrease in observable inputs related to the valuation of such securities.