Fair Value of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013
Fair Value Disclosures [Line Items]
Gross transfers out of Level 2 into Level 3		$ 21.7	[1]
Gross transfers out of Level 3 into Level 1	$ 100.0	$ 100.0

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