Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Details) $ in Thousands	3 Months Ended
Mar. 31, 2021 USD ($)
Goodwill [Line Items]
Beginning balance	$ 4,395,918
Goodwill of acquired businesses	79,924
Goodwill disposed of relating to sales of businesses	0
Foreign currency translation adjustments during the year	(2,800)
Ending balance	4,473,042
Retail
Goodwill [Line Items]
Beginning balance	2,650,470
Goodwill of acquired businesses	83,301
Goodwill disposed of relating to sales of businesses	0
Foreign currency translation adjustments during the year	(3,469)
Ending balance	2,730,302
National Programs
Goodwill [Line Items]
Beginning balance	1,091,122
Goodwill of acquired businesses	(1,337)
Goodwill disposed of relating to sales of businesses	0
Foreign currency translation adjustments during the year	669
Ending balance	1,090,454
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	483,057
Goodwill of acquired businesses	(2,040)
Goodwill disposed of relating to sales of businesses	0
Foreign currency translation adjustments during the year	0
Ending balance	481,017
Services
Goodwill [Line Items]
Beginning balance	171,269
Goodwill of acquired businesses	0
Goodwill disposed of relating to sales of businesses	0
Foreign currency translation adjustments during the year	0
Ending balance	$ 171,269

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10)(1))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

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-Publisher FASB
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Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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Amount of divestiture of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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