Revenues (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Document Period End Date	Mar. 31, 2019
Investment Income, Nonoperating	$ 1,081,000	$ 601,000
Other Income	736,000	522,000
Revenues	619,280,000	501,461,000
Contract with Customer, Asset and Liability [Table Text Block]	311,137,000		$ 265,994,000
Contract with Customer, Liability	39,600,000		37,000,000.0
Contract with Customer, Liability, Noncurrent	17,700,000		16,500,000
Contract with Customer, Liability	$ 57,283,000		53,496,000
Deferred Revenue, Revenue Recognized	not significant
Deferred Costs	$ 16,300,000		13,200,000
Operating Segments [Member]
Segment Reporting Information [Line Items]
Investment Income, Nonoperating	677,000
Other Income	0
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Investment Income, Nonoperating	677,000	413,000
Revenues	488,000	370,000
Retail [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Investment Income, Nonoperating	0	1,000
Other Income	592,000
Revenues	389,231,000	278,402,000
National Programs [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Investment Income, Nonoperating	316,000	114,000
Other Income	37,000
Revenues	109,560,000	112,475,000
Wholesale Brokerage [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Investment Income, Nonoperating	40,000	0
Other Income	107,000
Revenues	70,509,000	66,148,000
Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Investment Income, Nonoperating	48,000	73,000
Other Income	0
Revenues	49,492,000	$ 44,066,000
Core Commission Revenue [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	418,539,000
Core Commission Revenue [Member] | Retail [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	287,200,000
Core Commission Revenue [Member] | National Programs [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	77,178,000
Core Commission Revenue [Member] | Wholesale Brokerage [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	54,173,000
Core Commission Revenue [Member] | Services [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	0
Core Commission Revenue [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	(12,000)
Fee Revenue [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	135,789,000
Fee Revenue [Member] | Retail [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	43,877,000
Fee Revenue [Member] | National Programs [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	30,246,000
Fee Revenue [Member] | Wholesale Brokerage [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	12,510,000
Fee Revenue [Member] | Services [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	49,444,000
Fee Revenue [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	(288,000)
Incentive Commission Revenue [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	44,178,000
Incentive Commission Revenue [Member] | Retail [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	42,775,000
Incentive Commission Revenue [Member] | National Programs [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	919,000
Incentive Commission Revenue [Member] | Wholesale Brokerage [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	373,000
Incentive Commission Revenue [Member] | Services [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	0
Incentive Commission Revenue [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	111,000
Profit-Sharing Contingent Commission Revenue [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	15,322,000
Profit-Sharing Contingent Commission Revenue [Member] | Retail [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	11,547,000
Profit-Sharing Contingent Commission Revenue [Member] | National Programs [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	860,000
Profit-Sharing Contingent Commission Revenue [Member] | Wholesale Brokerage [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	2,915,000
Profit-Sharing Contingent Commission Revenue [Member] | Services [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	0
Profit-Sharing Contingent Commission Revenue [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	0
Guaranteed Supplemental Commission Revenue [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	3,635,000
Guaranteed Supplemental Commission Revenue [Member] | Retail [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	3,240,000
Guaranteed Supplemental Commission Revenue [Member] | National Programs [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	4,000
Guaranteed Supplemental Commission Revenue [Member] | Wholesale Brokerage [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	391,000
Guaranteed Supplemental Commission Revenue [Member] | Services [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	0
Guaranteed Supplemental Commission Revenue [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Fees and Commissions, Other	0
Cost to Obtain [Member]
Segment Reporting Information [Line Items]
Deferred Costs	3,400,000
Capitalized Contract Cost, Amortization	300,000
Cost to Fulfill [Member]
Segment Reporting Information [Line Items]
Deferred Costs	62,400,000		$ 69,800,000
Capitalized Contract Cost, Amortization	$ 7,400,000