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Marketable Securities - Investments Classified By Contractual Maturity Date (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Available-for-sale Securities, Debt Maturities, Single Maturity Date [Abstract]    
Due in one year or less $ 1,226,896 $ 1,722,553
Due after one year through three years 497,961 851,547
Total $ 1,724,857 $ 2,574,100