Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 430	$ 349	$ 909	$ 1,844
Other income, net	221	2,217	2,414	3,364
Total revenues	770,305	673,962	2,312,933	1,971,264
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	509,072		1,521,438
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	220,936		618,455
Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	16,624		91,983
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	17,618		63,163
Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	5,404		14,571
Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	284,536		905,470
Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	109,915		307,314
Retail | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	15,460		88,219
Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	8,705		32,848
Retail | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	4,454		12,383
National Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	133,649		369,703
National Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	49,779		125,329
National Programs | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	444		1,443
National Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	6,499		23,833
National Programs | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	549		1,316
Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	90,862		246,233
Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	17,938		51,574
Wholesale Brokerage | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	720		2,321
Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	2,414		6,482
Wholesale Brokerage | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	401		872
Services | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other
Services | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	43,732		135,590
Services | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0
Services | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0
Services | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	236	19	271	143
Other income, net	115		930
Total revenues	423,421	359,473	1,347,435	1,119,524
Operating Segments | National Programs
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	136	205	423	597
Other income, net	9		185
Total revenues	191,065	168,018	522,232	451,098
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	35	45	120	141
Other income, net	98		332
Total revenues	112,468	101,239	307,934	267,790
Operating Segments | Services
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0	0	3	0
Total revenues	43,732	$ 43,497	135,593	$ 130,879
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	23		92
Other income, net	(1)		967
Total revenues	(381)		(261)
Corporate, Non-Segment | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	25		32
Corporate, Non-Segment | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ (428)		$ (1,352)