Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 1,099	$ 2,811	$ 5,780
Other income, net	2,777	4,456	1,654
Total revenues	3,051,398	2,613,375	2,392,171
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	2,010,012	1,751,414
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	833,030	674,009
Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	103,249	93,557
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	82,226	70,934
Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	19,005	16,194
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	278	163	149
Other income, net	993	1,251
Total revenues	1,767,938	1,472,766	1,367,261
Operating Segments | Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1,198,129	1,054,619
Operating Segments | Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	414,937	275,900
Operating Segments | Retail | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	98,254	89,920
Operating Segments | Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	38,895	35,785
Operating Segments | Retail | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	16,452	15,128
Operating Segments | National Programs
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	550	756	1,397
Other income, net	192	42
Total revenues	701,850	610,640	518,384
Operating Segments | National Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	488,787	422,916
Operating Segments | National Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	173,790	159,337
Operating Segments | National Programs | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1,653	549
Operating Segments | National Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	35,259	27,278
Operating Segments | National Programs | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1,619	(238)
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	155	184	178
Other income, net	627	452
Total revenues	403,417	352,797	310,087
Operating Segments | Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	323,054	273,878
Operating Segments | Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	67,233	66,051
Operating Segments | Wholesale Brokerage | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	3,342	3,057
Operating Segments | Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	8,072	7,871
Operating Segments | Wholesale Brokerage | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	934	1,304
Operating Segments | Services
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	3	0	139
Other income, net	0	0
Total revenues	178,860	174,012	$ 193,781
Operating Segments | Services | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Operating Segments | Services | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	178,857	174,012
Operating Segments | Services | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Operating Segments | Services | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Operating Segments | Services | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	113	1,708
Other income, net	965	2,711
Total revenues	(667)	3,160
Corporate, Non-Segment | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	42	1
Corporate, Non-Segment | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(1,787)	(1,291)
Corporate, Non-Segment | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other		31
Corporate, Non-Segment | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Corporate, Non-Segment | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ 0	$ 0