Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 250	$ 1,161
Other	989	831
Total revenues	815,276	698,495
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	539,555	460,033
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	183,989	151,913
Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	59,285	56,759
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	25,950	23,942
Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	5,258	3,856
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	36	765
Total revenues	(381)	444
Corporate, Non-Segment | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	7
Corporate, Non-Segment | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(424)	(322)
Corporate, Non-Segment | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other		1
Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	357,459	315,888
Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	87,051	57,483
Retail | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	57,347	52,802
Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	15,735	16,095
Retail | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	4,615	4,432
Retail | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	18	83
Other	690	706
Total revenues	522,915	447,489
National Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	110,745	88,582
National Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	34,329	34,659
National Programs | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	788	197
National Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	8,316	5,711
National Programs | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	394	(1,013)
National Programs | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	152	263
Other	169	13
Total revenues	154,893	128,412
Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	71,344	55,563
Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	16,062	15,641
Wholesale Brokerage | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1,150	3,759
Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1,899	2,136
Wholesale Brokerage | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	249	437
Wholesale Brokerage | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	41	50
Other	130	112
Total revenues	90,875	77,698
Services | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Services | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	46,971	44,452
Services | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Services | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Services | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Services | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	3	0
Total revenues	$ 46,974	$ 44,452