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Fair Value Measurement - Additional Information (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Fair Value Disclosures [Abstract] |  |
Restricted investments | $ 24.2 | $ 20.9

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- Definition The aggregate value of all restricted investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-12C.9(c)) -URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611225-123010 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-12.6(c)) -URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611133-123010 + Details
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