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Fair Value Measurement - Summary of Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | $ 372,953 | $ 392,615
Gross Unrealized Gains (Losses) | (7,005) | (1,936)
Recorded Basis | 365,948 | 390,679
Investments Current [Member] | Cash and Cash Equivalents [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 30 | 221
Recorded Basis | 30 | 221
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 | 30 | 121
Recorded Basis | 30 | 121
Investments Current [Member] | Cash and Cash Equivalents [Member] | U.S. Treasuries [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost |  | 100
Recorded Basis |  | 100
Investments Current [Member] | Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 417 | 674
Gross Unrealized Gains (Losses) | (1) |
Recorded Basis | 416 | 674
Investments Current [Member] | Investments [Member] | U.S. Treasuries [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 347 | 199
Recorded Basis | 347 | 199
Investments Current [Member] | Investments [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 20 | 400
Recorded Basis | 20 | 400
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 | 50 | 65
Gross Unrealized Gains (Losses) | (1) |
Recorded Basis | 49 | 65
Investments Current [Member] | Investments [Member] | Municipal Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost |  | 10
Recorded Basis |  | 10
Investments Noncurrent [Member] | Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 1,710 | 1,208
Gross Unrealized Gains (Losses) | (23) | (9)
Recorded Basis | 1,687 | 1,199
Investments Noncurrent [Member] | Investments [Member] | U.S. Treasuries [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 794 | 202
Gross Unrealized Gains (Losses) | (3) | (2)
Recorded Basis | 791 | 200
Investments Noncurrent [Member] | Investments [Member] | Corporate Bonds [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 422 | 419
Gross Unrealized Gains (Losses) | (7) | (2)
Recorded Basis | 415 | 417
Investments Noncurrent [Member] | Investments [Member] | Mortgage Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 484 | 577
Gross Unrealized Gains (Losses) | (13) | (5)
Recorded Basis | 471 | 572
Investments Noncurrent [Member] | Investments [Member] | Asset Backed Securities [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 10 | 10
Recorded Basis | 10 | 10
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 370,796 | 390,512
Gross Unrealized Gains (Losses) | (6,981) | (1,927)
Recorded Basis | 363,815 | 388,585
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 | 419 | 16,018
Recorded Basis | 419 | 16,018
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 | 44,304 | 45,833
Gross Unrealized Gains (Losses) | (283) | (143)
Recorded Basis | 44,021 | 45,690
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 | 185,116 | 184,808
Gross Unrealized Gains (Losses) | (3,739) | (953)
Recorded Basis | 181,377 | 183,855
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 | 45,548 | 38,168
Gross Unrealized Gains (Losses) | (908) | (222)
Recorded Basis | 44,640 | 37,946
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 | 472
Gross Unrealized Gains (Losses) |  | (14)
Recorded Basis | 50 | 458
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 | 89,426 | 86,240
Gross Unrealized Gains (Losses) | (2,026) | (595)
Recorded Basis | 87,400 | 85,645
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 | 75 |
Gross Unrealized Gains (Losses) | (1) |
Recorded Basis | 74 |
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Supranational [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost | 4,765 |
Gross Unrealized Gains (Losses) | (24) |
Recorded Basis | 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 | 1,093 |
Recorded Basis | $ 1,093 |
Investments Noncurrent [Member] | Restricted Cash and Investments [Member] | Commercial Paper [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Cost |  | 18,973
Recorded Basis |  | $ 18,973

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