Business Combinations - Purchase Price Allocation for Current Year Acquisitions and Adjustments Made for Prior Year Acquisitions (Details) - USD ($) $ in Thousands				3 Months Ended			9 Months Ended
	Sep. 01, 2021	May 01, 2021	Jan. 01, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2021
Business Combination Separately Recognized Transactions [Line Items]
Cash paid							$ 224,141
Common stock issued				$ 4,892	$ 5,120	$ 10,000	4,892
Other payable							27,609
Recorded earn-out payable							25,163
Net assets acquired							281,805
Maximum potential earn- out payable							61,252
O'Leary Insurances (O'Leary)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired							137,648
O'Leary Insurances (O'Leary) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Jan. 01, 2021
Cash paid			$ 117,408
Common stock issued			4,892
Recorded earn-out payable			15,348
Net assets acquired			137,648
Maximum potential earn- out payable			$ 30,575
Piper Jordan LLC (Piper)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired							54,679
Piper Jordan LLC (Piper) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		May 01, 2021
Cash paid		$ 43,428
Other payable		1,397
Recorded earn-out payable		9,854
Net assets acquired		54,679
Maximum potential earn- out payable		$ 15,000
Berkshire Insurance Group, Inc. (Berkshire)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired							41,500
Berkshire Insurance Group, Inc. (Berkshire) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition	Sep. 01, 2021
Cash paid	$ 41,500
Net assets acquired	$ 41,500
AGIS Network Inc. (AGIS)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired							36,056
AGIS Network Inc. (AGIS) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition	Sep. 01, 2021
Cash paid	$ 11,203
Other payable	24,114
Recorded earn-out payable	739
Net assets acquired	36,056
Maximum potential earn- out payable	$ 12,289
Other Acquisitions
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired							11,922
Other Acquisitions | Various
Business Combination Separately Recognized Transactions [Line Items]
Cash paid							10,602
Other payable							2,098
Recorded earn-out payable							(778)
Net assets acquired							11,922
Maximum potential earn- out payable							$ 3,388