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Financial instruments - Aggregate Estimated Fair Value of Investments with Unrealized Losses (Detail) - U.S. Treasury Securities [Member]
$ in Thousands
Dec. 31, 2017
USD ($)
Schedule of Available-for-sale Securities [Line Items]  
Period of continuous unrealized loss, 12 months or less, Fair value $ 15,983
Period of continuous unrealized loss, 12 months or less, Gross unrealized losses (27)
Period of continuous unrealized loss, Greater than 12 months, Fair value 40,070
Period of continuous unrealized loss, Greater than 12 months, Gross unrealized losses $ (43)