Revenues - Schedule of Disaggregated by Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	$ 333	$ 1,525	$ 1,495	$ 2,606
Other income	316	762	1,147	1,498
Revenues	598,806	575,219	1,297,302	1,194,499
Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	117		882
Other income	0		0
Revenues	(206)		238
Retail | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	41	17	124	17
Other income	227		933
Revenues	312,562	310,738	760,051	699,969
National Programs | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	129	380	392	696
Other income	7		21
Revenues	154,667	131,817	283,080	241,377
Wholesale Brokerage | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	46	43	97	83
Other income	82		193
Revenues	88,853	81,171	166,551	151,680
Services | Operating Segments
Disaggregation Of Revenue [Line Items]
Investment Income, Nonoperating	0	34	0	82
Other income	0		0
Revenues	42,930	$ 50,797	87,382	$ 100,289
Core Commission Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	397,366		864,153
Core Commission Revenue | Retail
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	221,674		540,218
Core Commission Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	107,392		198,159
Core Commission Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	68,300		125,776
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	162,499		312,450
Fee Revenue | Corporate, Non-Segment
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	(323)		(644)
Fee Revenue | Retail
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	65,583		123,224
Fee Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	37,453		70,013
Fee Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	16,856		32,475
Fee Revenue | Services
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	42,930		87,382
Incentive Commissions Revenue
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	15,333		67,300
Incentive Commissions Revenue | Retail
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	14,706		64,694
Incentive Commissions Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	111		222
Incentive Commissions Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	516		2,384
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	18,653		42,595
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	6,926		23,021
Profit-Sharing Contingent Commission Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	9,269		14,980
Profit-Sharing Contingent Commission Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	2,458		4,594
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	4,306		8,162
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	3,405		7,837
Guaranteed Supplemental Commission Revenue | National Programs
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	306		(707)
Guaranteed Supplemental Commission Revenue | Wholesale Brokerage
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	595		1,032
Guaranteed Supplemental Commission Revenue | Services
Disaggregation Of Revenue [Line Items]
Fees and Commissions, Other	$ 0		$ 0