Business Combinations - Purchase Price Allocation for Current Year Acquisitions and Adjustments Made for Prior Year Acquisitions (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)
Business Combination Separately Recognized Transactions [Line Items]
Cash paid		$ 291,610
Common Stock Issued	$ 20,000	20,000
Other payable		9,462
Recorded earn-out payable	71,464	71,464
Net assets acquired	392,536	392,536
Maximum potential earn- out payable	141,953	141,953
Smith Insurance Associates, Inc. (Smith)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	22,833	$ 22,833
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	2,704
Net assets acquired	22,833	22,833
Maximum potential earn- out payable	4,550	4,550
Donald P. Pipino Company, LTD (Pipino)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	26,376	$ 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	9,821
Net assets acquired	26,376	26,376
Maximum potential earn- out payable	12,996	12,996
Cossio Insurance Agency (Cossio)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	15,710	$ 15,710
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	1,710	1,710
Net assets acquired	15,710	15,710
Maximum potential earn- out payable	2,000	2,000
Medval LLC (Medval)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	30,890	$ 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	1,684
Net assets acquired	30,890	30,890
Maximum potential earn- out payable	2,500	2,500
United Development Systems, Inc. (United)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	22,643	$ 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	3,268
Net assets acquired	22,643	22,643
Maximum potential earn- out payable	8,625	8,625
Twinbrook Insurance Brokerage, Inc. (Twinbrook)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	28,216	$ 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	1,565
Net assets acquired	28,216	28,216
Maximum potential earn- out payable	5,073	5,073
Innovative Risk Solutions, Inc. (IRS)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	34,382	$ 34,382
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	5,482	5,482
Net assets acquired	34,382	34,382
Maximum potential earn- out payable	9,000	9,000
Other Acquisitions | Various
Business Combination Separately Recognized Transactions [Line Items]
Cash paid		27,405
Other payable		1,291
Recorded earn-out payable	4,172	4,172
Net assets acquired	32,868	32,868
Maximum potential earn- out payable	9,404	9,404
WBR Insurance Agency, LLC et al (WBR)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	13,067	$ 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	2,197
Net assets acquired	13,067	13,067
Maximum potential earn- out payable	4,575	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	$ 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	768
Net assets acquired	14,268	14,268
Maximum potential earn- out payable	6,730	6,730
CKP Insurance, LLC (CKP)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	151,283	$ 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	38,093
Net assets acquired	151,283	151,283
Maximum potential earn- out payable	$ 76,500	$ 76,500