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Investments - Contractual Maturities of Debt Securities (Detail) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale debt securities, Fair Value/Carrying Value | $ 61,063 | $ 62,493
Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 2,800 |
One to two years | 10,122 |
Two to three years | 9,898 |
Three to five years | 2,742 |
Available-for-sale debt securities, Fair Value/Carrying Value | 25,562 | 32,543
Municipal Fixed-Rate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
One to two years | 287 |
Two to three years | 1,128 |
Available-for-sale debt securities, Fair Value/Carrying Value | 1,415 | 2,882
Asset-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 3,112 |
One to two years | 657 |
Two to three years | 714 |
Three to five years | 2,244 |
Five to ten years | 445 |
More than ten years | 1,183 |
Available-for-sale debt securities, Fair Value/Carrying Value | 8,355 | 6,526
Mortgage/Agency-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Two to three years | 425 |
Three to five years | 1,561 |
Five to ten years | 525 |
More than ten years | 2,852 |
Available-for-sale debt securities, Fair Value/Carrying Value | 5,363 | 5,509
U.S. Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
One to two years | 11,716 |
Two to three years | 5,565 |
Three to five years | 2,492 |
Available-for-sale debt securities, Fair Value/Carrying Value | 19,773 | 14,303
Foreign Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
One to two years | 285 |
Two to three years | 310 |
Available-for-sale debt securities, Fair Value/Carrying Value | $ 595 | $ 730

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- Definition Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803 + Details
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