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Financial instruments - Aggregate Estimated Fair Value of Investments with Unrealized Losses (Detail) - U.S. Treasury Securities [Member] - USD ($)
$ in Thousands
Sep. 30, 2016
Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]    
Period of continuous unrealized loss, 12 months or less, Fair value $ 38,250 $ 71,411
Period of continuous unrealized loss, 12 months or less, Gross unrealized losses (18) (290)
Period of continuous unrealized loss, Greater than 12 months, Fair value 0 0
Period of continuous unrealized loss, Greater than 12 months, Gross unrealized losses $ 0 $ 0