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Fair Value Measurements - Roll-forward of fair value of deferred compensation liabilities categorized as Level 3 in fair value hierarchy (Detail) - Deferred Compensation Liabilities - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] |  |  |  |
Opening balance | $ 35,034 | $ 21,375 | $ 26,390 | $ 10,943
Additions (net of redemption) | 840 | 792 | 6,799 | 10,367
Change in fair value of deferred compensation plan liabilities | (159) | 1,085 | 2,526 | 1,942
Closing balance | $ 35,715 | $ 23,252 | $ 35,715 | $ 23,252

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- Definition Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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