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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, Foreign Government Bonds and Variable Rate Demand Notes (Detail) - USD ($) $ in Thousands | Mar. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale-securities, Fair Value/Carrying Value | $ 198,337 | $ 190,723
Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 31,566 |
One to two years | 20,265 |
Two to three years | 19,036 |
Three to five years | 1,406 |
Available-for-sale-securities, Fair Value/Carrying Value | 72,273 | 66,412
Municipal Fixed-Rate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 8,237 |
One to two years | 1,540 |
Three to five years | 1,182 |
Available-for-sale-securities, Fair Value/Carrying Value | 10,959 | 11,774
Asset-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
One to two years | 1,017 |
Two to three years | 2,078 |
Three to five years | 7,416 |
Five to ten years | 1,173 |
More than ten years | 144 |
Available-for-sale-securities, Fair Value/Carrying Value | 11,828 | 10,206
Mortgage/Agency-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
One to two years | 976 |
Two to three years | 726 |
Five to ten years | 1,896 |
More than ten years | 8,715 |
Available-for-sale-securities, Fair Value/Carrying Value | 12,313 | 13,004
U.S. Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 750 |
One to two years | 6,365 |
Two to three years | 8,569 |
Three to five years | 12,363 |
Available-for-sale-securities, Fair Value/Carrying Value | 28,047 | 29,767
Foreign Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 1,400 |
One to two years | 1,329 |
Two to three years | 550 |
Available-for-sale-securities, Fair Value/Carrying Value | 3,279 | 3,730
Variable Rate Demand Notes [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Three to five years | 1,790 |
Five to ten years | 2,000 |
More than ten years | 6,715 |
Available-for-sale-securities, Fair Value/Carrying Value | $ 10,505 | $ 11,855

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