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Investment in equity method investees-Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Equity Method Investment Summarized Financial Information Income Statement Abstract      
Equity Method Investments $ 225,625us-gaap_EquityMethodInvestments $ 197,006us-gaap_EquityMethodInvestments  
Income Loss From Equity Method Investments $ 39,578us-gaap_IncomeLossFromEquityMethodInvestments $ 42,873us-gaap_IncomeLossFromEquityMethodInvestments $ 73,478us-gaap_IncomeLossFromEquityMethodInvestments