Business Combinations - Purchase Price Allocation for Current Year Acquisitions and Adjustments Made for Prior Year Acquisitions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Combination Separately Recognized Transactions [Line Items]
Cash paid	$ 722,515	$ 356,260	$ 934,928
Common Stock Issued	30,120	20,000	100,000
Other payable	9,130	12,135	5,462
Recorded earn-out payable	131,397	82,872	77,377
Net assets acquired	893,162	471,267	1,117,767
Maximum potential earn- out payable	$ 273,450	162,023	147,668
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
Net assets acquired	80,015
Maximum potential earn- out payable	14,650
Texas All Risk General Agency, Inc. et al (Texas Risk)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 10,980
Texas All Risk General Agency, Inc. et al (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
Net assets acquired	10,980
Maximum potential earn- out payable	1,150
The Colonial Group, Inc. et al (Colonial)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 37,141
The Colonial Group, Inc. et al (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	7,577
Net assets acquired	37,141
Maximum potential earn- out payable	10,150
RLA Insurance Intermediaries, LLC (RLA)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 54,969
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	11,687
Net assets acquired	54,969
Maximum potential earn- out payable	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	$ 23,770
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	4,129
Net assets acquired	23,770
Maximum potential earn- out payable	5,400
LP Insurance Services, LLC (LP)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 149,660
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	23,394
Net assets acquired	149,660
Maximum potential earn- out payable	75,850
First Resource, Inc. (First)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 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
Net assets acquired	14,926
Maximum potential earn- out payable	5,800
Buiten & Associates, LLC (Buiten)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 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
Net assets acquired	46,848
Maximum potential earn- out payable	$ 14,175
Amity Insurance, 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
Net assets acquired	18,880
Maximum potential earn- out payable	4,060
Frank E. Neal & Co., Inc. (Neal)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 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
Net assets acquired	41,786
Maximum potential earn- out payable	10,325
BrookStone Insurance Group, LLC (BrookStone)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 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
Net assets acquired	13,088
Maximum potential earn- out payable	1,878
Other Acquisitions | Various
Business Combination Separately Recognized Transactions [Line Items]
Cash paid	14,888	36,665	30,293
Other payable	490	2,391	1,367
Recorded earn-out payable	8,498	9,026	5,896
Net assets acquired	23,876	48,082	37,556
Maximum potential earn- out payable	$ 12,337	$ 14,454	12,998
VAS GenPar, LLC (VAS) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition	Oct. 01, 2020
Cash paid	$ 114,249
Common Stock Issued	15,000
Recorded earn-out payable	23,274
Net assets acquired	152,523
Maximum potential earn- out payable	48,000
Bright & Associates, Inc. (Bright)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 17,639
Bright & Associates, Inc. (Bright) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition	Oct. 01, 2020
Cash paid	$ 12,528
Other payable	1,257
Recorded earn-out payable	3,854
Net assets acquired	17,639
Maximum potential earn- out payable	$ 5,775
J.E. Brown & Associates Insurance Services, Inc. (J.E. Brown) | Wholesale Brokerage
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition	Oct. 01, 2020
Cash paid	$ 33,331
Other payable	1,030
Recorded earn-out payable	5,947
Net assets acquired	40,308
Maximum potential earn- out payable	10,425
CoverHound, Inc. and CyberPolicy, Inc. (CoverHound)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 28,195
CoverHound, Inc. and CyberPolicy, Inc. (CoverHound) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition	Nov. 01, 2020
Cash paid	$ 27,595
Other payable	600
Net assets acquired	28,195
MAJ Companies, Ltd. (MAJ)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 21,378
MAJ Companies, Ltd. (MAJ) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition	Dec. 01, 2020
Cash paid	$ 19,072
Other payable	300
Recorded earn-out payable	2,006
Net assets acquired	21,378
Maximum potential earn- out payable	$ 6,475
South & Western General Agency, Inc. (South & Western) | Wholesale Brokerage
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition	Dec. 01, 2020
Cash paid	$ 69,673
Other payable	1,193
Recorded earn-out payable	7,294
Net assets acquired	78,160
Maximum potential earn- out payable	18,000
Berry Insurance Group, Inc. (Berry)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 39,020
Berry Insurance Group, Inc. (Berry) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition	Dec. 31, 2020
Cash paid	$ 35,326
Recorded earn-out payable	3,694
Net assets acquired	39,020
Maximum potential earn- out payable	$ 6,500
Smith Insurance Associates, Inc. (Smith) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Feb. 01, 2019
Cash paid		$ 20,129
Recorded earn-out payable		2,704
Net assets acquired		22,833
Maximum potential earn- out payable		4,550
Donald P. Pipino Company, LTD (Pipino)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired		$ 26,376
Donald P. Pipino Company, LTD (Pipino) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Feb. 01, 2019
Cash paid		$ 16,420
Other payable		135
Recorded earn-out payable		9,821
Net assets acquired		26,376
Maximum potential earn- out payable		12,996
AGA Enterprises, LLC d/b/a Cossio Insurance Agency (Cossio)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired		$ 14,696
AGA Enterprises, LLC d/b/a Cossio Insurance Agency (Cossio) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Mar. 01, 2019
Cash paid		$ 13,990
Other payable		10
Recorded earn-out payable		696
Net assets acquired		14,696
Maximum potential earn- out payable		2,000
Medval, LLC (Medval)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired		$ 30,890
Medval, LLC (Medval) | Services
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Mar. 01, 2019
Cash paid		$ 29,106
Other payable		100
Recorded earn-out payable		1,684
Net assets acquired		30,890
Maximum potential earn- out payable		2,500
United Development Systems, Inc. (United)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired		$ 22,643
United Development Systems, Inc. (United) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		May 01, 2019
Cash paid		$ 18,987
Other payable		388
Recorded earn-out payable		3,268
Net assets acquired		22,643
Maximum potential earn- out payable		8,625
Twinbrook Insurance Brokerage, Inc. (Twinbrook)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired		$ 28,216
Twinbrook Insurance Brokerage, Inc. (Twinbrook) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Jun. 01, 2019
Cash paid		$ 26,251
Other payable		400
Recorded earn-out payable		1,565
Net assets acquired		28,216
Maximum potential earn- out payable		5,073
Innovative Risk Solutions, Inc. (IRS)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired		$ 35,009
Innovative Risk Solutions, Inc. (IRS) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Jul. 01, 2019
Cash paid		$ 26,435
Other payable		2,465
Recorded earn-out payable		6,109
Net assets acquired		35,009
Maximum potential earn- out payable		9,000
WBR Insurance Agency, LLC et al (WBR)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired		$ 13,067
WBR Insurance Agency, LLC et al (WBR) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Aug. 01, 2019
Cash paid		$ 10,667
Other payable		203
Recorded earn-out payable		2,197
Net assets acquired		13,067
Maximum potential earn- out payable		4,575
West Ridge Insurance Agency, Inc. d/b/a Yozell Associates (Yozell)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired		$ 14,268
West Ridge Insurance Agency, Inc. d/b/a Yozell Associates (Yozell) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Aug. 01, 2019
Cash paid		$ 13,030
Other payable		470
Recorded earn-out payable		768
Net assets acquired		14,268
Maximum potential earn- out payable		6,730
CKP Insurance, LLC (CKP)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired		$ 151,283
CKP Insurance, LLC (CKP) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Aug. 01, 2019
Cash paid		$ 89,190
Common Stock Issued		20,000
Other payable		4,000
Recorded earn-out payable		38,093
Net assets acquired		151,283
Maximum potential earn- out payable		$ 76,500
Poole Professional Ltd. Insurance Agents and Brokers et al (Poole) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Oct. 01, 2019
Cash paid		$ 32,358
Other payable		75
Recorded earn-out payable		4,556
Net assets acquired		36,989
Maximum potential earn- out payable		6,850
VerHagen Glendenning & Walker LLP (VGW)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired		$ 26,915
VerHagen Glendenning & Walker LLP (VGW) | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition		Oct. 01, 2019
Cash paid		$ 23,032
Other payable		1,498
Recorded earn-out payable		2,385
Net assets acquired		26,915
Maximum potential earn- out payable		$ 8,170
Opus
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired			$ 22,984
Opus | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Feb. 01, 2018
Cash paid			$ 20,400
Other payable			200
Recorded earn-out payable			2,384
Net assets acquired			22,984
Maximum potential earn- out payable			3,600
Kerxton
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired			$ 16,746
Kerxton | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Mar. 01, 2018
Cash paid			$ 13,176
Other payable			1,490
Recorded earn-out payable			2,080
Net assets acquired			16,746
Maximum potential earn- out payable			2,920
ADG
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired			$ 47,575
ADG | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			May 01, 2018
Cash paid			$ 29,471
Other payable			559
Recorded earn-out payable			17,545
Net assets acquired			47,575
Maximum potential earn- out payable			20,000
Servco
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired			$ 77,179
Servco | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Jun. 01, 2018
Cash paid			$ 76,245
Recorded earn-out payable			934
Net assets acquired			77,179
Maximum potential earn- out payable			7,000
Tower Hill
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired			$ 21,488
Tower Hill | National Programs
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Jul. 01, 2018
Cash paid			$ 20,300
Recorded earn-out payable			1,188
Net assets acquired			21,488
Maximum potential earn- out payable			7,700
HSR
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired			$ 22,123
HSR | National Programs
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Jul. 01, 2018
Cash paid			$ 20,132
Recorded earn-out payable			1,991
Net assets acquired			22,123
Maximum potential earn- out payable			9,000
PDA
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired			$ 24,843
PDA | Services
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Jul. 01, 2018
Cash paid			$ 15,025
Recorded earn-out payable			9,818
Net assets acquired			24,843
Maximum potential earn- out payable			17,975
F&I
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired			$ 54,471
F&I | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Sep. 01, 2018
Cash paid			$ 44,940
Other payable			410
Recorded earn-out payable			9,121
Net assets acquired			54,471
Maximum potential earn- out payable			19,500
Rodman
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired			$ 35,102
Rodman | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Nov. 01, 2018
Cash paid			$ 31,121
Other payable			261
Recorded earn-out payable			3,720
Net assets acquired			35,102
Maximum potential earn- out payable			9,850
Hays
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired			$ 724,600
Hays | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Nov. 16, 2018
Cash paid			$ 605,000
Common Stock Issued			100,000
Recorded earn-out payable			19,600
Net assets acquired			724,600
Maximum potential earn- out payable			25,000
Dealer Associates
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired			$ 33,100
Dealer Associates | Retail
Business Combination Separately Recognized Transactions [Line Items]
Effective date of acquisition			Dec. 01, 2018
Cash paid			$ 28,825
Other payable			1,175
Recorded earn-out payable			3,100
Net assets acquired			33,100
Maximum potential earn- out payable			$ 12,125