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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | $ 361,468 | $ 372,953
Gross Unrealized Gains (Losses) | 3,896 | (7,005)
Recorded Basis | 365,364 | 365,948
Investments Current [Member] | Cash and Cash Equivalents [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 35,526 | 30
Recorded Basis | 35,526 | 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 | 35,526 | 30
Recorded Basis | 35,526 | 30
Investments Current [Member] | Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 82,155 | 417
Gross Unrealized Gains (Losses) | 435 | (1)
Recorded Basis | 82,590 | 416
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 53,493 | 20
Gross Unrealized Gains (Losses) | 156 |
Recorded Basis | 53,649 | 20
Investments Current [Member] | Investments [Member] | Asset Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 14,017 |
Gross Unrealized Gains (Losses) | (13) |
Recorded Basis | 14,004 |
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 4,500 | 347
Gross Unrealized Gains (Losses) | 9 |
Recorded Basis | 4,509 | 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 | 7,408 | 50
Gross Unrealized Gains (Losses) | 282 | (1)
Recorded Basis | 7,690 | 49
Investments Current [Member] | Investments [Member] | Mortgage Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 2,737 |
Gross Unrealized Gains (Losses) | 1 |
Recorded Basis | 2,738 |
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] | 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] | 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] | Restricted Cash and Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 243,787 | 370,796
Gross Unrealized Gains (Losses) | 3,461 | (6,981)
Recorded Basis | 247,248 | 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 | 29,046 | 419
Recorded Basis | 29,046 | 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 | 98,481 | 185,116
Gross Unrealized Gains (Losses) | 1,931 | (3,739)
Recorded Basis | 100,412 | 181,377
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 | 303 | 75
Gross Unrealized Gains (Losses) |  | (1)
Recorded Basis | 303 | 74
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 | 16,906 | 44,304
Gross Unrealized Gains (Losses) | 21 | (283)
Recorded Basis | 16,927 | 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 | 27,885 | 45,548
Gross Unrealized Gains (Losses) | 642 | (908)
Recorded Basis | 28,527 | 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 | 62,930 | 89,426
Gross Unrealized Gains (Losses) | 837 | (2,026)
Recorded Basis | 63,767 | 87,400
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 | 4,770 | 4,765
Gross Unrealized Gains (Losses) | 30 | (24)
Recorded Basis | 4,800 | 4,741
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 | 3,466 | 1,093
Recorded Basis | $ 3,466 | 1,093
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
Recorded Basis |  | $ 50

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