Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 698,495	$ 619,280
Investment income	1,161	1,081
Amortization	26,391	26,192
Depreciation	6,021	6,035
Interest expense	15,291	15,198
Income before income taxes	205,253	148,520
Total assets	7,582,345	6,806,289	$ 7,622,821
Capital expenditures	16,458	23,186
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues	447,489	389,231
Investment income	83	0
Amortization	16,287	15,444
Depreciation	2,036	1,624
Interest expense	21,798	21,153
Income before income taxes	128,662	93,841
Total assets	6,375,532	5,773,290
Capital expenditures	2,499	3,136
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	128,412	109,560
Investment income	263	316
Amortization	6,365	6,596
Depreciation	1,905	1,590
Interest expense	4,682	5,354
Income before income taxes	29,645	19,316
Total assets	3,054,411	2,852,824
Capital expenditures	1,513	4,568
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	77,698	70,509
Investment income	50	40
Amortization	2,349	2,849
Depreciation	431	404
Interest expense	1,711	1,327
Income before income taxes	17,686	15,569
Total assets	1,514,243	1,292,406
Capital expenditures	1,227	1,189
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	44,452	49,492
Investment income	0	48
Amortization	1,390	1,303
Depreciation	351	284
Interest expense	1,091	872
Income before income taxes	10,080	9,177
Total assets	475,168	480,108
Capital expenditures	267	109
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues	444	488
Investment income	765	677
Amortization	0	0
Depreciation	1,298	2,133
Interest expense	(13,991)	(13,508)
Income before income taxes	19,180	10,617
Total assets	(3,837,009)	(3,592,339)
Capital expenditures	$ 10,952	$ 14,184