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Investments - Carrying Value and Amortized Cost of Company's Investments Summarized by Major Security Type (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Estimated Fair Value	$ 150,239	$ 271,924
Equity securities, Amortized Cost	3,149
Equity securities, Unrealized Losses	(1,967)
Equity securities, Estimated Fair Value	1,182
Debt and equity securities, Amortized Cost	153,712	272,331
Debt and equity securities, Unrealized Gains	6	1
Debt and equity securities, Unrealized Losses	(2,297)	(408)
Debt and equity securities, Estimated Fair Value	151,421	271,924
U.S. Treasury notes
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost		32,976
Debt securities, Unrealized Losses		(12)
Debt securities, Estimated Fair Value		32,964
Government sponsored enterprise securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost		10,082
Debt securities, Unrealized Losses		(10)
Debt securities, Estimated Fair Value		10,072
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	124,521	138,650
Debt securities, Unrealized Gains	5	1
Debt securities, Unrealized Losses	(318)	(321)
Debt securities, Estimated Fair Value	124,208	138,330
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	26,042	90,623
Debt securities, Unrealized Gains	1
Debt securities, Unrealized Losses	(12)	(65)
Debt securities, Estimated Fair Value	$ 26,031	$ 90,558

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