Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 530,850	$ 475,646	$ 1,505,498	$ 1,407,031
Investment income	719	491	2,051	1,085
Amortization	21,637	21,435	62,961	64,402
Depreciation	5,259	5,489	16,410	17,242
Interest expense	8,963	9,393	28,686	28,949
Income before income taxes	142,500	124,419	361,851	343,388
Total assets	5,802,806	7,414,277	5,802,806	7,414,277	$ 5,747,550
Capital expenditures	9,469	4,049	28,859	12,897
Operating Segments
Segment Reporting Information [Line Items]
Investment income	485		1,380
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	261,087	234,483	772,806	712,739
Investment income	1	3	2	6
Amortization	10,754	10,540	31,498	31,704
Depreciation	1,236	1,262	3,746	3,975
Interest expense	7,468	7,216	21,377	23,918
Income before income taxes	61,224	54,950	175,984	151,783
Total assets	4,651,080	4,246,422	4,651,080	4,246,422
Capital expenditures	1,467	844	5,828	3,384
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	143,519	127,718	374,434	342,576
Investment income	138	124	384	279
Amortization	6,736	6,913	19,383	20,664
Depreciation	1,346	1,412	4,087	4,975
Interest expense	6,147	8,304	20,019	27,257
Income before income taxes	46,707	32,203	91,809	68,127
Total assets	2,933,465	5,026,918	2,933,465	5,026,918
Capital expenditures	2,420	1,357	7,313	3,885
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	78,009	71,574	219,725	208,812
Investment income	44	0	125	0
Amortization	2,878	2,845	8,537	8,621
Depreciation	385	471	1,238	1,438
Interest expense	1,233	1,515	4,039	4,803
Income before income taxes	23,614	21,219	55,544	56,569
Total assets	1,272,795	1,258,783	1,272,795	1,258,783
Capital expenditures	895	214	1,762	1,606
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	48,105	41,491	138,038	122,397
Investment income	51	72	160	224
Amortization	1,269	1,137	3,543	3,412
Depreciation	393	402	1,183	1,191
Interest expense	846	876	2,023	2,783
Income before income taxes	6,988	7,910	23,889	22,480
Total assets	448,027	432,331	448,027	432,331
Capital expenditures	366	364	798	856
Other
Segment Reporting Information [Line Items]
Total revenues	130	380	495	20,507
Investment income	485	292	1,380	576
Amortization	0	0	0	1
Depreciation	1,899	1,942	6,156	5,663
Interest expense	(6,731)	(8,518)	(18,772)	(29,812)
Income before income taxes	3,967	8,137	14,625	44,429
Total assets	(3,502,561)	(3,550,177)	(3,502,561)	(3,550,177)
Capital expenditures	$ 4,321	$ 1,270	$ 13,158	$ 3,166