Segment Information - Summarized Financial Information Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 618,683	$ 530,850	$ 1,813,182	$ 1,505,498
Investment income	1,668	719	4,274	2,051
Amortization	26,272	21,637	78,418	62,961
Depreciation	5,815	5,259	17,516	16,410
Interest expense	16,314	8,963	47,805	28,686
Income before income taxes	151,838	142,500	423,880	361,851
Total assets	7,680,664	5,802,806	7,680,664	5,802,806	$ 6,863,968	$ 6,688,668
Capital expenditures	12,184	9,469	47,358	28,859
Operating Segments | Retail
Segment Reporting Information [Line Items]
Revenues	337,712	261,087	1,037,681	772,806
Investment income	28	1	45	2
Amortization	15,821	10,754	46,622	31,498
Depreciation	1,897	1,236	5,440	3,746
Interest expense	22,287	7,468	64,641	21,377
Income before income taxes	51,133	61,224	189,191	175,984
Total assets	6,155,670	4,651,080	6,155,670	4,651,080
Capital expenditures	2,266	1,467	7,810	5,828
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Revenues	142,854	143,519	384,231	374,434
Investment income	375	138	1,071	384
Amortization	6,264	6,736	19,217	19,383
Depreciation	1,779	1,346	4,990	4,087
Interest expense	3,557	6,147	13,134	20,019
Income before income taxes	46,629	46,707	106,188	91,809
Total assets	3,315,656	2,933,465	3,315,656	2,933,465
Capital expenditures	1,276	2,420	7,786	7,313
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Revenues	87,109	78,009	238,789	219,725
Investment income	48	44	131	125
Amortization	2,797	2,878	8,490	8,537
Depreciation	422	385	1,251	1,238
Interest expense	1,105	1,233	3,692	4,039
Income before income taxes	28,908	23,614	67,421	55,544
Total assets	1,406,709	1,272,795	1,406,709	1,272,795
Capital expenditures	768	895	2,470	1,762
Operating Segments | Services
Segment Reporting Information [Line Items]
Revenues	50,106	48,105	150,395	138,038
Investment income	35	51	117	160
Amortization	1,390	1,269	4,089	3,543
Depreciation	315	393	902	1,183
Interest expense	1,164	846	3,239	2,023
Income before income taxes	14,775	6,988	33,220	23,889
Total assets	469,292	448,027	469,292	448,027
Capital expenditures	209	366	540	798
Segment Reconciling Items
Segment Reporting Information [Line Items]
Revenues	902	130	2,086	495
Investment income	1,182	485	2,910	1,380
Amortization	0	0	0	0
Depreciation	1,402	1,899	4,933	6,156
Interest expense	(11,799)	(6,731)	(36,901)	(18,772)
Income before income taxes	10,393	3,967	27,860	14,625
Total assets	(3,666,663)	(3,502,561)	(3,666,663)	(3,502,561)
Capital expenditures	$ 7,665	$ 4,321	$ 28,752	$ 13,158