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Investments - Carrying Value and Amortized Cost of Company's Investments Summarized by Major Security Type (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |
Debt securities, Estimated Fair Value | $ 182,673 | $ 271,924
Equity securities, Amortized Cost | 3,149 |
Equity securities, Unrealized Losses | (247) |
Equity securities, Estimated Fair Value | 2,902 |
Debt and equity securities, Amortized Cost | 186,332 | 272,331
Debt and equity securities, Unrealized Gains | 6 | 1
Debt and equity securities, Unrealized Losses | (763) | (408)
Debt and equity securities, Estimated Fair Value | 185,575 | 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 | 139,304 | 138,650
Debt securities, Unrealized Gains |  | 1
Debt securities, Unrealized Losses | (502) | (321)
Debt securities, Estimated Fair Value | 138,802 | 138,330
Commercial paper |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Debt securities, Amortized Cost | 43,879 | 90,623
Debt securities, Unrealized Gains | 6 |
Debt securities, Unrealized Losses | (14) | (65)
Debt securities, Estimated Fair Value | $ 43,871 | $ 90,558

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