Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total revenues	$ 466,305	$ 446,518	$ 931,385	$ 870,691
Investment income	351	502	594	920
Amortization	21,347	21,610	42,967	43,220
Depreciation	5,655	5,354	11,753	10,672
Interest expense	9,874	9,837	19,556	19,734
Income before income taxes	108,002	109,077	218,969	211,626
Total assets	5,279,009	5,206,877	5,279,009	5,206,877	$ 5,262,734
Capital expenditures	5,816	4,692	8,848	8,944
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	238,969	234,560	478,256	466,748
Investment income	1	7	3	28
Amortization	10,517	10,893	21,164	21,882
Depreciation	1,324	1,616	2,713	3,253
Interest expense	8,126	9,986	16,702	20,389
Income before income taxes	47,978	49,150	96,833	99,602
Total assets	4,090,094	3,593,733	4,090,094	3,593,733
Capital expenditures	1,404	2,172	2,540	3,221
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	113,675	108,820	214,858	209,890
Investment income	81	239	155	487
Amortization	6,847	6,982	13,751	14,090
Depreciation	1,604	2,007	3,563	3,936
Interest expense	8,918	11,461	18,953	24,051
Income before income taxes	23,397	22,245	35,924	36,048
Total assets	2,723,177	2,556,293	2,723,177	2,556,293
Capital expenditures	1,448	1,314	2,528	3,246
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	71,991	61,287	137,238	114,701
Investment income	0	1	0	4
Amortization	2,845	2,591	5,776	5,033
Depreciation	477	488	967	984
Interest expense	1,613	685	3,288	932
Income before income taxes	20,085	16,287	35,350	30,849
Total assets	1,204,184	1,018,689	1,204,184	1,018,689
Capital expenditures	1,014	322	1,392	914
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	41,580	41,752	80,906	78,320
Investment income	72	83	152	147
Amortization	1,137	1,140	2,275	2,205
Depreciation	390	472	789	959
Interest expense	946	1,327	1,907	2,563
Income before income taxes	8,449	6,906	14,570	11,958
Total assets	396,165	327,957	396,165	327,957
Capital expenditures	342	276	492	481
Other
Segment Reporting Information [Line Items]
Total revenues	90	99	20,127	1,032
Investment income	197	172	284	254
Amortization	1	4	1	10
Depreciation	1,860	771	3,721	1,540
Interest expense	(9,729)	(13,622)	(21,294)	(28,201)
Income before income taxes	8,093	14,489	36,292	33,169
Total assets	(3,134,611)	(2,289,795)	(3,134,611)	(2,289,795)
Capital expenditures	$ 1,608	$ 608	$ 1,896	$ 1,082