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Marketable Investments - Summary of Company's Fair Value Hierarchy (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Jun. 30, 2013
Federal Agency and Corporate Obligations [Member]
Dec. 31, 2012
Federal Agency and Corporate Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents   $ 6.1 $ 32.1
Original maturities 90 days