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Investments - Contractual Maturities of Debt Securities (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Available-for-sale debt securities, Fair Value/Carrying Value | $ 89,068 | $ 62,493
Corporate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 11,391 |
One to two years | 18,406 |
Two to three years | 10,639 |
Three to five years | 8,383 |
Available-for-sale debt securities, Fair Value/Carrying Value | 48,819 | 32,543
Municipal Fixed-Rate Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 490 |
One to two years | 737 |
Two to three years | 210 |
Three to five years | 946 |
Available-for-sale debt securities, Fair Value/Carrying Value | 2,383 | 2,882
Asset-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less than one year | 4,520 |
One to two years | 2,459 |
Two to three years | 2,669 |
Three to five years | 1,631 |
More than ten years | 2,011 |
Available-for-sale debt securities, Fair Value/Carrying Value | 13,290 | 6,526
Mortgage/Agency-Backed Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Three to five years | 2,046 |
Five to ten years | 974 |
More than ten years | 5,816 |
Available-for-sale debt securities, Fair Value/Carrying Value | 8,836 | 5,509
U.S. Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
One to two years | 6,149 |
Two to three years | 2,491 |
Three to five years | 5,869 |
Available-for-sale debt securities, Fair Value/Carrying Value | 14,509 | 14,303
Foreign Government Bonds [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Two to three years | 1,231 |
Available-for-sale debt securities, Fair Value/Carrying Value | $ 1,231 | $ 730

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- Definition Fair value of available-for-sale debt securities maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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://www.xbrl.org/2003/role/presentationRef -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 + Details
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- Definition Fair value of available-for-sale debt securities maturing after the tenth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 + Details
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