Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 1,161	$ 1,081
Other income	831	736
Revenues	698,495	619,280
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	765	677
Other income	0	0
Revenues	444	488
Retail | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	83	0
Other income	706	592
Revenues	447,489	389,231
National Programs | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	263	316
Other income	13	37
Revenues	128,412	109,560
Wholesale Brokerage | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	50	40
Other income	112	107
Revenues	77,698	70,509
Services | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0	48
Other income	0	0
Revenues	44,452	49,492
Core Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	460,033	418,539
Core Commission Revenue | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other		(12)
Core Commission Revenue | Retail
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	315,888	287,200
Core Commission Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	88,582	77,178
Core Commission Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	55,563	54,173
Core Commission Revenue | Services
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Fee Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	151,913	135,789
Fee Revenue | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(322)	(288)
Fee Revenue | Retail
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	57,483	43,877
Fee Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	34,659	30,246
Fee Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	15,641	12,510
Fee Revenue | Services
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	44,452	49,444
Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	56,759	44,178
Incentive Commissions Revenue | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1	111
Incentive Commissions Revenue | Retail
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	52,802	42,775
Incentive Commissions Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	197	919
Incentive Commissions Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	3,759	373
Incentive Commissions Revenue | Services
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Profit-Sharing Contingent Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	23,942	15,322
Profit-Sharing Contingent Commission Revenue | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Profit-Sharing Contingent Commission Revenue | Retail
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	16,095	11,547
Profit-Sharing Contingent Commission Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	5,711	860
Profit-Sharing Contingent Commission Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	2,136	2,915
Profit-Sharing Contingent Commission Revenue | Services
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Guaranteed Supplemental Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	3,856	3,635
Guaranteed Supplemental Commission Revenue | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	0	0
Guaranteed Supplemental Commission Revenue | Retail
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	4,432	3,240
Guaranteed Supplemental Commission Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(1,013)	4
Guaranteed Supplemental Commission Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	437	391
Guaranteed Supplemental Commission Revenue | Services
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ 0	$ 0