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Goodwill and Intangible Assets - Changes in goodwill by reporting unit (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2021 | Dec. 31, 2020
Goodwill [Roll Forward] |  |
Opening balance | $ 1,695,688 | $ 1,574,466
Goodwill relating to acquisitions consummated during the period | 0 | 123,595
Impact of measurement period adjustments | (108) | (5,653)
Effect of exchange rate fluctuations | (9,896) | 3,280
Closing balance | 1,685,684 | 1,695,688
BCMI |  |
Goodwill [Roll Forward] |  |
Opening balance | 420,172 | 417,213
Goodwill relating to acquisitions consummated during the period | 0 | 2,559
Impact of measurement period adjustments | (3) | (542)
Effect of exchange rate fluctuations | (3,061) | 942
Closing balance | 417,108 | 420,172
CGRLH |  |
Goodwill [Roll Forward] |  |
Opening balance | 607,574 | 555,130
Goodwill relating to acquisitions consummated during the period | 0 | 52,612
Impact of measurement period adjustments | (28) | (1,372)
Effect of exchange rate fluctuations | (3,726) | 1,204
Closing balance | 603,820 | 607,574
HMS |  |
Goodwill [Roll Forward] |  |
Opening balance | 667,942 | 602,123
Goodwill relating to acquisitions consummated during the period | 0 | 68,424
Impact of measurement period adjustments | (77) | (3,739)
Effect of exchange rate fluctuations | (3,109) | 1,134
Closing balance | $ 664,756 | $ 667,942

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