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Subsequent events - Additional Information (Detail) (USD $)
 1 Months Ended 12 Months Ended 1 Months Ended
Dec. 31, 2012
Dec. 31, 2012
Jan. 31, 2013
Pending Litigation [Member]
Dividends Payable [Line Items]      
Cash dividend payable amount per share $ 0.11    
Cash dividend payable $ 1,500,000    
Date of payment of dividend Feb. 22, 2013    
Date of record of dividend Feb. 08, 2013    
Date of announcement of dividend Jan. 24, 2013    
Contingency damages amount   30,000,000 30,000,000
Claim amount including interest and costs     140,000,000
Initial claim amount including interest and costs     $ 253,000,000
Case filed date     February 2009