Summary of Unrealized Gains and Losses Related to Cash Equivalents and Investments in Marketable Securities (Parenthetical) (Detail) (Available-for-sale Securities, USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Available-for-sale Securities
Schedule Of Available For Sale Securities [Line Items]
Interest income receivable	$ 0.2	$ 0.4