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Financial Instruments - Investments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019
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Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Total investments at fair value | $ 243,651 | $ 185,488
Fair Value Measurements on Recurring Basis [Member] |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Total investments at fair value | 243,651 | 185,488
Fair Value Measurements on Recurring Basis [Member] | Municipal Bonds [Member] |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Total investments at fair value | 1,227 | 1,364
Fair Value Measurements on Recurring Basis [Member] | Non-governmental Debt Securities [Member] |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Total investments at fair value | 205,646 | 165,819
Fair Value Measurements on Recurring Basis [Member] | Treasury and Federal Agencies [Member] |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Total investments at fair value | 36,778 | 18,305
Fair Value Measurements on Recurring Basis [Member] | Level 2 [Member] |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Total investments at fair value | 243,651 | 185,488
Fair Value Measurements on Recurring Basis [Member] | Level 2 [Member] | Municipal Bonds [Member] |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Total investments at fair value | 1,227 | 1,364
Fair Value Measurements on Recurring Basis [Member] | Level 2 [Member] | Non-governmental Debt Securities [Member] |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Total investments at fair value | 205,646 | 165,819
Fair Value Measurements on Recurring Basis [Member] | Level 2 [Member] | Treasury and Federal Agencies [Member] |  |
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items] |  |
Total investments at fair value | $ 36,778 | $ 18,305

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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