Acquisitions and Dispositions of Businesses - Completed Acquisitions Narrative (Details) - acquisition	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Apr. 06, 2020	Jan. 31, 2020	Jan. 03, 2020	Jan. 01, 2020
Business Acquisition [Line Items]
Number of business acquired under business combination	0	1	0	6
Farmington Administrative Services LLC
Business Acquisition [Line Items]
Percentage of capital acquired					100.00%
Cytelligence Inc.
Business Acquisition [Line Items]
Percentage of capital acquired						100.00%
Cover Wallet Inc.
Business Acquisition [Line Items]
Percentage of capital acquired							100.00%
TRIUM GmbH Insurance Broker
Business Acquisition [Line Items]
Percentage of capital acquired								100.00%
Assimedia SA
Business Acquisition [Line Items]
Percentage of capital acquired								100.00%
Apollo Conseil et Courtage
Business Acquisition [Line Items]
Percentage of capital acquired								100.00%