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Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Investments [Line Items]
Investments	$ 624,866	$ 508,578
Commercial paper
Schedule Of Investments [Line Items]
Investments	42,465	75,362
Corporate debt securities
Schedule Of Investments [Line Items]
Investments	541,694	414,815
Domestic and foreign government agencies debt securities
Schedule Of Investments [Line Items]
Investments	$ 40,707	$ 18,401

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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