Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 473,187	$ 466,305	$ 974,648	$ 931,385
Investment income	731	351	1,332	594
Amortization	20,785	21,347	41,324	42,967
Depreciation	5,599	5,655	11,151	11,753
Interest expense	10,052	9,874	19,723	19,556
Income before income taxes	100,910	108,002	219,351	218,969
Total assets	5,701,557	5,279,009	5,701,557	5,279,009	$ 5,747,550
Capital expenditures	9,639	5,816	19,390	8,848
Operating Segments
Segment Reporting Information [Line Items]
Investment income	482		895
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	233,317	238,969	511,719	478,256
Investment income	0	1	1	3
Amortization	10,502	10,517	20,744	21,164
Depreciation	1,273	1,324	2,510	2,713
Interest expense	7,112	8,126	13,909	16,702
Income before income taxes	44,361	47,978	114,760	96,833
Total assets	4,668,251	4,090,094	4,668,251	4,090,094
Capital expenditures	1,954	1,404	4,361	2,540
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	118,440	113,675	230,915	214,858
Investment income	132	81	246	155
Amortization	6,324	6,847	12,647	13,751
Depreciation	1,354	1,604	2,741	3,563
Interest expense	6,376	8,918	13,872	18,953
Income before income taxes	24,324	23,397	45,102	35,924
Total assets	2,844,485	2,723,177	2,844,485	2,723,177
Capital expenditures	2,215	1,448	4,893	2,528
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	75,568	71,991	141,716	137,238
Investment income	81	0	81	0
Amortization	2,822	2,845	5,659	5,776
Depreciation	426	477	853	967
Interest expense	1,371	1,613	2,806	3,288
Income before income taxes	20,547	20,085	31,930	35,350
Total assets	1,267,207	1,204,184	1,267,207	1,204,184
Capital expenditures	447	1,014	867	1,392
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	45,867	41,580	89,933	80,906
Investment income	36	72	109	152
Amortization	1,137	1,137	2,274	2,275
Depreciation	379	390	790	789
Interest expense	583	946	1,177	1,907
Income before income taxes	8,085	8,449	16,901	14,570
Total assets	419,876	396,165	419,876	396,165
Capital expenditures	147	342	432	492
Other
Segment Reporting Information [Line Items]
Total revenues	(5)	90	365	20,127
Investment income	482	197	895	284
Amortization	0	1	0	1
Depreciation	2,167	1,860	4,257	3,721
Interest expense	(5,390)	(9,729)	(12,041)	(21,294)
Income before income taxes	3,593	8,093	10,658	36,292
Total assets	(3,498,262)	(3,134,611)	(3,498,262)	(3,134,611)
Capital expenditures	$ 4,876	$ 1,608	$ 8,837	$ 1,896