Business Combinations - Purchase Price Allocation for Current Year Acquisitions and Adjustments Made for Prior Year Acquisitions (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)
Business Combination Separately Recognized Transactions [Line Items]
Cash paid		$ 291,424
Common Stock Issued	$ 10,000	10,000
Other payable		2,165
Recorded earn-out payable	74,754	74,754
Net assets acquired	378,343	378,343
Maximum potential earn- out payable	133,640	$ 133,640
Special Risk Insurance Managers Ltd. (Special Risk) | National Programs
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Jan. 01, 2020
Cash paid		$ 70,156
Recorded earn-out payable	8,173	8,173
Net assets acquired	78,329	78,329
Maximum potential earn- out payable	14,650	14,650
Texas All Risk General Agency, Inc. (Texas Risk)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	10,980	$ 10,980
Texas All Risk General Agency, Inc. (Texas Risk) | Wholesale Brokerage
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Jan. 01, 2020
Cash paid		$ 10,511
Other payable		159
Recorded earn-out payable	310	310
Net assets acquired	10,980	10,980
Maximum potential earn- out payable	1,150	1,150
The Colonial Group, Inc. (Colonial)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	35,203	$ 35,203
The Colonial Group, Inc. (Colonial) | Wholesale Brokerage
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Mar. 01, 2020
Cash paid		$ 29,037
Other payable		527
Recorded earn-out payable	5,639	5,639
Net assets acquired	35,203	35,203
Maximum potential earn- out payable	10,150	10,150
RLA Insurance Intermediaries, LLC (RLA)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	53,252	$ 53,252
RLA Insurance Intermediaries, LLC (RLA) | Wholesale Brokerage
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Mar. 01, 2020
Cash paid		$ 42,496
Other payable		786
Recorded earn-out payable	9,970	9,970
Net assets acquired	53,252	53,252
Maximum potential earn- out payable	22,500	22,500
Dealer Financial Services of N.C., LLC d/b/a The Sterling Group (Sterling)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	22,481	$ 22,481
Dealer Financial Services of N.C., LLC d/b/a The Sterling Group (Sterling) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Apr. 01, 2020
Cash paid		$ 19,341
Other payable		300
Recorded earn-out payable	2,840	2,840
Net assets acquired	22,481	22,481
Maximum potential earn- out payable	5,400	5,400
LP Insurance Services, LLC (LP)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	169,259	$ 169,259
LP Insurance Services, LLC (LP) | National Programs
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		May 01, 2020
Cash paid		$ 115,948
Common Stock Issued		10,000
Other payable		318
Recorded earn-out payable	42,993	42,993
Net assets acquired	169,259	169,259
Maximum potential earn- out payable	75,850	75,850
Other Acquisitions | Various
Business Combination Separately Recognized Transactions [Line Items]
Cash paid		3,935
Other payable		75
Recorded earn-out payable	4,829	4,829
Net assets acquired	8,839	8,839
Maximum potential earn- out payable	$ 3,940	$ 3,940