Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 229	$ 333	$ 479	$ 1,495
Other income, net	1,204	316	2,193	1,147
Total revenues	727,352	598,806	1,542,628	1,297,302
Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	472,812		1,012,367
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	213,530		397,518
Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	16,073		75,359
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	19,594		45,544
Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	3,910		9,168
Retail | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	263,474		620,932
Retail | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	110,348		197,399
Retail | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	15,412		72,760
Retail | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	8,408		24,143
Retail | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	3,315		7,930
National Programs | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	125,310		236,055
National Programs | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	41,221		75,550
National Programs | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	210		998
National Programs | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	9,017		17,333
National Programs | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	373		767
Wholesale Brokerage | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	84,027		155,371
Wholesale Brokerage | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	17,574		33,636
Wholesale Brokerage | Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	451		1,601
Wholesale Brokerage | Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	2,169		4,068
Wholesale Brokerage | Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	222		471
Services | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other			0
Services | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	44,887		91,858
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
Operating Segments | Retail
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	17	41	35	124
Other income, net	125		815
Total revenues	401,099	312,562	924,014	760,051
Operating Segments | National Programs
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	134	129	286	392
Other income, net	9		178
Total revenues	176,274	154,667	331,167	283,080
Operating Segments | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	44	46	85	97
Other income, net	104		234
Total revenues	104,591	88,853	195,466	166,551
Operating Segments | Services
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0	0	3	0
Total revenues	44,887	$ 42,930	91,861	$ 87,382
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	34		70
Other income, net	966		966
Total revenues	501		120
Corporate, Non-Segment | Base Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1		9
Corporate, Non-Segment | Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ (500)		$ (925)