Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash equivalents original maturity period	Three months or less
Payment of tax withholdings on certain stock-based compensation	$ 14,408	$ 12,387	$ 11,718

Amount of cash outflow to satisfy an employee's income tax withholding obligation as part of a net-share settlement of a share-based award.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 25
-Paragraph 19A
-URI http://asc.fasb.org/extlink&oid=96949009&loc=SL79513924-113897

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585

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Balance Type:	credit
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