Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 349	$ 1,668	$ 1,844	$ 4,274
Other income	2,217	(348)	3,364	1,150
Revenues	673,962	618,683	1,971,264	1,813,182
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	80		963
Other income	1,964		1,964
Revenues	1,735		1,973
Retail | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	19	28	143	45
Other income	123		1,056
Revenues	359,473	337,712	1,119,524	1,037,681
National Programs | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	205	375	597	1,071
Other income	11		32
Revenues	168,018	142,854	451,098	384,231
Wholesale Brokerage | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	45	48	141	131
Other income	119		312
Revenues	101,239	87,109	267,790	238,789
Services | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0	35	0	117
Other income	0		0
Revenues	43,497	$ 50,106	130,879	$ 150,395
Core Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	461,654		1,325,807
Core Commission Revenue | Retail
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	265,028		805,245
Core Commission Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	116,184		314,344
Core Commission Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	80,442		206,218
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	177,139		489,589
Fee Revenue | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(323)		(968)
Fee Revenue | Retail
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	70,440		193,664
Fee Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	45,722		115,736
Fee Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	17,803		50,278
Fee Revenue | Services
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	43,497		130,879
Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	14,468		81,767
Incentive Commissions Revenue | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	14		14
Incentive Commissions Revenue | Retail
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	13,913		78,607
Incentive Commissions Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	215		436
Incentive Commissions Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	326		2,710
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	13,739		56,334
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	6,359		29,380
Profit-Sharing Contingent Commission Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	5,499		20,478
Profit-Sharing Contingent Commission Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	1,881		6,476
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	4,396		12,559
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	3,591		11,429
Guaranteed Supplemental Commission Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	182		(525)
Guaranteed Supplemental Commission Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	623		1,655
Guaranteed Supplemental Commission Revenue | Services
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ 0		$ 0