Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating									$ 2,811	$ 5,780	$ 2,746
Other income									4,456	1,654	1,643
Total revenues	$ 642,111	$ 673,962	$ 598,806	$ 698,495	$ 578,989	$ 618,683	$ 575,219	$ 619,280	2,613,375	2,392,171	2,014,246
Core Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									1,751,414	1,574,480
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									674,009	646,766
Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									93,557	81,260
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									70,934	59,166
Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									16,194	23,065
Retail | Core Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									1,054,619	994,170
Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									275,900	246,135
Retail | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									89,920	80,505
Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									35,785	34,150
Retail | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									15,128	11,056
National Programs | Core Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									422,916	338,058
National Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									159,337	151,298
National Programs | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									549	(524)
National Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									27,278	17,517
National Programs | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									(238)	10,566
Wholesale Brokerage | Core Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									273,878	242,380
Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									66,051	56,852
Wholesale Brokerage | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									3,057	1,252
Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									7,871	7,499
Wholesale Brokerage | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									1,304	1,443
Services | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									174,012	193,641
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating									163	149	2
Other income									1,251	1,096
Total revenues									1,472,766	1,367,261	1,042,763
Operating Segments | National Programs
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating									756	1,397	506
Other income									42	72
Total revenues									610,640	518,384	494,463
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating									184	178	165
Other income									452	483
Total revenues									352,797	310,087	287,014
Operating Segments | Services
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating									0	139	205
Other income										1
Total revenues									174,012	193,781	$ 189,246
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating									1,708	3,917
Other income									2,711	2
Total revenues									3,160	2,658
Corporate, Non-Segment | Core Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									1	(128)
Corporate, Non-Segment | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									(1,291)	(1,160)
Corporate, Non-Segment | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other									$ 31	$ 27