Business Combinations - Purchase Price Allocation for Current Year Acquisitions and Adjustments Made for Prior Year Acquisitions (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Business Combination Separately Recognized Transactions [Line Items]
Cash paid	$ 153,291
Other payable	1,536
Recorded earn-out payable	13,978
Net assets acquired	168,816
Maximum potential earn- out payable	$ 49,590
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	$ 68,812
Recorded earn-out payable	4,884
Net assets acquired	73,696
Maximum potential earn- out payable	14,650
Texas All Risk General Agency, Inc. (Texas Risk)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 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	135
Recorded earn-out payable	310
Net assets acquired	10,980
Maximum potential earn- out payable	1,150
The Colonial Group, Inc. (Colonial)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 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	10
Recorded earn-out payable	5,639
Net assets acquired	35,203
Maximum potential earn- out payable	10,150
RLA Insurance Intermediaries, LLC (RLA)
Business Combination Separately Recognized Transactions [Line Items]
Net assets acquired	$ 46,233
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	100
Recorded earn-out payable	2,951
Net assets acquired	46,233
Maximum potential earn- out payable	22,500
Other Acquisitions | Various
Business Combination Separately Recognized Transactions [Line Items]
Cash paid	2,435
Other payable	1,291
Recorded earn-out payable	194
Net assets acquired	2,704
Maximum potential earn- out payable	$ 1,140