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. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020
Business Combination Separately Recognized Transactions [Line Items]
Cash paid				$ 402,358
Common Stock Issued	$ 5,120	$ 10,000	$ 20,000	15,120
Other payable				4,350
Recorded earn-out payable	96,845			96,845
Net assets acquired	518,673			518,673
Maximum potential earn- out payable	171,778			$ 171,778
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	9,859			9,859
Net assets acquired	80,015			80,015
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
First Resource, Inc. (First)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	14,926			$ 14,926
First Resource, Inc. (First) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition				Jul. 01, 2020
Cash paid				$ 10,700
Other payable				450
Recorded earn-out payable	3,776			3,776
Net assets acquired	14,926			14,926
Maximum potential earn- out payable	5,800			5,800
Buiten & Associates, LLC (Buiten)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	46,848			$ 46,848
Buiten & Associates, LLC (Buiten) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition				Aug. 01, 2020
Cash paid				$ 38,225
Other payable				1,175
Recorded earn-out payable	7,448			7,448
Net assets acquired	46,848			46,848
Maximum potential earn- out payable	14,175			$ 14,175
Amity Insurance Agency, Inc. (Amity) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition				Aug. 01, 2020
Cash paid				$ 14,820
Common Stock Issued				2,000
Other payable				200
Recorded earn-out payable	1,860			1,860
Net assets acquired	18,880			18,880
Maximum potential earn- out payable	4,060			4,060
Frank E. Neal & Co., Inc. (Neal)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	41,786			$ 41,786
Frank E. Neal & Co., Inc. (Neal) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition				Sep. 01, 2020
Cash paid				$ 32,589
Common Stock Issued				3,120
Other payable				345
Recorded earn-out payable	5,732			5,732
Net assets acquired	41,786			41,786
Maximum potential earn- out payable	10,325			10,325
BrookStone Insurance Group, LLC (BrookStone)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	13,088			$ 13,088
BrookStone Insurance Group, LLC (BrookStone) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition				Sep. 01, 2020
Cash paid				$ 12,030
Recorded earn-out payable	1,058			1,058
Net assets acquired	13,088			13,088
Maximum potential earn- out payable	1,878			1,878
Other Acquisitions | Various
Business Combination Separately Recognized Transactions [Line Items]
Cash paid				6,505
Other payable				90
Recorded earn-out payable	5,360			5,360
Net assets acquired	11,955			11,955
Maximum potential earn- out payable	$ 5,840			$ 5,840