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Investments - Contractual Maturities of Corporate Bonds, Municipal Fixed-Rate Bonds, Asset-Backed Bonds, Mortgage/Agency-Backed Bonds, U.S. Government Bonds and Foreign Government Bonds (Detail) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale-securities, Fair Value/Carrying Value | $ 140,025 | $ 190,723
Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 20,232 |
One to two years | 11,678 |
Two to three years | 10,029 |
Three to five years | 3,280 |
Available-for-sale-securities, Fair Value/Carrying Value | 45,219 | 66,412
Municipal Fixed-Rate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 2,250 |
One to two years | 724 |
Two to three years | 730 |
Three to five years | 1,170 |
Available-for-sale-securities, Fair Value/Carrying Value | 4,874 | 11,774
Asset-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
One to two years | 2,638 |
Two to three years | 2,235 |
Three to five years | 1,550 |
Five to ten years | 450 |
More than ten years | 912 |
Available-for-sale-securities, Fair Value/Carrying Value | 7,785 | 10,206
Mortgage/Agency-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Three to five years | 917 |
Five to ten years | 1,348 |
More than ten years | 5,065 |
Available-for-sale-securities, Fair Value/Carrying Value | 7,330 | 13,004
U.S. Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 8,903 |
Two to three years | 10,413 |
Three to five years | 1,578 |
Available-for-sale-securities, Fair Value/Carrying Value | 20,894 | 29,767
Foreign Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Two to three years | 728 |
Available-for-sale-securities, Fair Value/Carrying Value | $ 728 | $ 3,730

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