Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 642,111	$ 673,962	$ 598,806	$ 698,495	$ 578,989	$ 618,683	$ 575,219	$ 619,280	$ 2,613,375	$ 2,392,171	$ 2,014,246
Investment income									2,811	5,780	2,746
Amortization									108,523	105,298	86,544
Depreciation									26,276	23,417	22,834
Interest expense									58,973	63,660	40,580
Income before income taxes	130,916	$ 158,528	$ 129,401	$ 205,253	102,049	$ 151,838	$ 123,522	$ 148,520	624,099	525,929	462,462
Total assets	8,966,492				7,622,821				8,966,492	7,622,821	6,688,668
Capital expenditures									70,700	73,108	41,520
Operating Segments | Retail
Segment Reporting Information [Line Items]
Total revenues									1,472,766	1,367,261	1,042,763
Investment income									163	149	2
Amortization									67,315	63,146	44,386
Depreciation									9,071	7,390	5,289
Interest expense									85,968	87,295	35,969
Income before income taxes									262,245	222,875	217,845
Total assets	7,093,627				6,413,459				7,093,627	6,413,459	5,850,045
Capital expenditures									13,175	12,497	6,858
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues									610,640	518,384	494,463
Investment income									756	1,397	506
Amortization									27,166	25,482	25,954
Depreciation									8,658	6,791	5,486
Interest expense									20,597	16,690	26,181
Income before income taxes									182,892	143,737	117,375
Total assets	3,510,983				3,110,368				3,510,983	3,110,368	2,940,097
Capital expenditures									7,208	10,365	12,391
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues									352,797	310,087	287,014
Investment income									184	178	165
Amortization									8,481	11,191	11,391
Depreciation									1,948	1,674	1,628
Interest expense									10,281	4,756	5,254
Income before income taxes									93,593	82,739	70,171
Total assets	1,791,717				1,390,250				1,791,717	1,390,250	1,283,877
Capital expenditures									3,324	6,171	2,518
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues									174,012	193,781	189,246
Investment income									0	139	205
Amortization									5,561	5,479	4,813
Depreciation									1,424	1,229	1,558
Interest expense									4,142	4,404	2,869
Income before income taxes									27,994	40,337	34,508
Total assets	480,440				481,336				480,440	481,336	471,572
Capital expenditures									1,424	804	1,525
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total revenues									3,160	2,658	760
Investment income									1,708	3,917	1,868
Amortization									0	0	0
Depreciation									5,175	6,333	8,873
Interest expense									(62,015)	(49,485)	(29,693)
Income before income taxes									57,375	36,241	22,563
Total assets	$ (3,910,275)				$ (3,772,592)				(3,910,275)	(3,772,592)	(3,856,923)
Capital expenditures									$ 45,569	$ 43,271	$ 18,228