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Marketable investments - Additional information (Detail) (USD $)
In Millions, unless otherwise specified
 6 Months Ended
Jun. 30, 2013
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Dec. 31, 2012
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Jun. 30, 2013
ARS [Member]
Level 3 [Member]
Jun. 30, 2013
Minimum [Member]
Jun. 30, 2012
Minimum [Member]
Jun. 30, 2013
Maximum [Member]
Jun. 30, 2012
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Intervals for each security       7 days   35 days  
Interest rates on securities ranged       0.10% 0.10% 0.40% 0.50%
Weighted average interest rate     1.70%        
Weighted average discount rate     5.30%        
Weighted average holding period     8 years 8 months 12 days        
Unrealized loss recorded in accumulated other comprehensive income (loss) $ 2.1 $ 2.0