Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Details) $ in Thousands	6 Months Ended
Jun. 30, 2021 USD ($)
Goodwill [Line Items]
Beginning balance	$ 4,395,918
Goodwill of acquired businesses	121,828
Goodwill disposed of relating to sales of businesses	(3,050)
Foreign currency translation adjustments during the year	(726)
Ending balance	4,513,970
Retail
Goodwill [Line Items]
Beginning balance	2,650,470
Goodwill of acquired businesses	122,612
Goodwill disposed of relating to sales of businesses	(3,050)
Foreign currency translation adjustments during the year	(2,421)
Ending balance	2,767,611
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	1,695
Ending balance	1,091,480
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	483,057
Goodwill of acquired businesses	553
Goodwill disposed of relating to sales of businesses	0
Foreign currency translation adjustments during the year	0
Ending balance	483,610
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
-URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
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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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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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