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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 | Dec. 31, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale-securities, Fair Value/Carrying Value | $ 118,038 | $ 190,723
Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 12,021 |
One to two years | 9,145 |
Two to three years | 7,345 |
Three to five years | 4,032 |
Available-for-sale-securities, Fair Value/Carrying Value | 32,543 | 66,412
Municipal Fixed-Rate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 891 |
One to two years | 826 |
Two to three years | 212 |
Three to five years | 953 |
Available-for-sale-securities, Fair Value/Carrying Value | 2,882 | 11,774
Asset-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 143 |
One to two years | 2,367 |
Two to three years | 2,245 |
Three to five years | 810 |
Five to ten years | 158 |
More than ten years | 803 |
Available-for-sale-securities, Fair Value/Carrying Value | 6,526 | 10,206
Mortgage/Agency-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Three to five years | 356 |
Five to ten years | 1,144 |
More than ten years | 4,009 |
Available-for-sale-securities, Fair Value/Carrying Value | 5,509 | 13,004
U.S. Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 3,073 |
One to two years | 5,960 |
Two to three years | 3,568 |
Three to five years | 1,702 |
Available-for-sale-securities, Fair Value/Carrying Value | 14,303 | 29,767
Foreign Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Two to three years | 730 |
Available-for-sale-securities, Fair Value/Carrying Value | $ 730 | $ 3,730

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