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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Sep. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | $ 350,141 | $ 372,953
Gross Unrealized Gains (Losses) | 3,617 | (7,005)
Recorded Basis | 353,758 | 365,948
Investments Current [Member] | Cash and Cash Equivalents [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 6,121 | 30
Recorded Basis | 6,121 | 30
Investments Current [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 6,121 | 30
Recorded Basis | 6,121 | 30
Investments Current [Member] | Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 101,434 | 417
Gross Unrealized Gains (Losses) | 314 | (1)
Recorded Basis | 101,748 | 416
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 70,922 | 20
Gross Unrealized Gains (Losses) | 36 |
Recorded Basis | 70,958 | 20
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 18,642 | 347
Gross Unrealized Gains (Losses) | (27) |
Recorded Basis | 18,615 | 347
Investments Current [Member] | Investments [Member] | U.S. Government Agency Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 8,910 | 50
Gross Unrealized Gains (Losses) | 313 | (1)
Recorded Basis | 9,223 | 49
Investments Current [Member] | Investments [Member] | Mortgage Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 2,943 |
Gross Unrealized Gains (Losses) | (8) |
Recorded Basis | 2,935 |
Investments Current [Member] | Investments [Member] | Asset Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 17 |
Recorded Basis | 17 |
Investments Noncurrent [Member] | Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost |  | 1,710
Gross Unrealized Gains (Losses) |  | (23)
Recorded Basis |  | 1,687
Investments Noncurrent [Member] | Investments [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost |  | 422
Gross Unrealized Gains (Losses) |  | (7)
Recorded Basis |  | 415
Investments Noncurrent [Member] | Investments [Member] | U.S. Treasuries [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost |  | 794
Gross Unrealized Gains (Losses) |  | (3)
Recorded Basis |  | 791
Investments Noncurrent [Member] | Investments [Member] | Mortgage Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost |  | 484
Gross Unrealized Gains (Losses) |  | (13)
Recorded Basis |  | 471
Investments Noncurrent [Member] | Investments [Member] | Asset Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost |  | 10
Recorded Basis |  | 10
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 242,586 | 370,796
Gross Unrealized Gains (Losses) | 3,303 | (6,981)
Recorded Basis | 245,889 | 363,815
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Money Market Funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 19,085 | 419
Gross Unrealized Gains (Losses) | (2) |
Recorded Basis | 19,083 | 419
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 101,533 | 185,116
Gross Unrealized Gains (Losses) | 1,793 | (3,739)
Recorded Basis | 103,326 | 181,377
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Treasuries [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 18,397 | 44,304
Gross Unrealized Gains (Losses) | 24 | (283)
Recorded Basis | 18,421 | 44,021
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | U.S. Government Agency Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 29,780 | 45,548
Gross Unrealized Gains (Losses) | 947 | (908)
Recorded Basis | 30,727 | 44,640
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Mortgage Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 66,012 | 89,426
Gross Unrealized Gains (Losses) | 507 | (2,026)
Recorded Basis | 66,519 | 87,400
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Asset Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 116 | 75
Gross Unrealized Gains (Losses) | 1 | (1)
Recorded Basis | 117 | 74
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Mutual Funds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 2,844 | 1,093
Recorded Basis | 2,844 | 1,093
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Supranational Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 4,769 | 4,765
Gross Unrealized Gains (Losses) | 33 | (24)
Recorded Basis | 4,802 | 4,741
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Municipal Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 50 | 50
Recorded Basis | $ 50 | $ 50

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