Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total revenues	$ 446,518	$ 419,447	$ 870,691	$ 823,745
Investment income	502	260	920	480
Amortization	21,610	21,623	43,220	43,248
Depreciation	5,354	5,237	10,672	10,420
Interest expense	9,837	9,671	19,734	19,522
Income before income taxes	109,077	100,914	211,626	194,692
Total assets	5,206,877	5,020,837	5,206,877	5,020,837	$ 5,004,479
Capital expenditures	4,692	4,870	8,944	8,597
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	234,560	222,721	466,748	441,065
Investment income	7	21	28	43
Amortization	10,893	11,185	21,882	22,119
Depreciation	1,616	1,634	3,253	3,276
Interest expense	9,986	10,562	20,389	20,720
Income before income taxes	49,150	48,455	99,602	95,464
Total assets	3,593,733	3,423,263	3,593,733	3,423,263
Capital expenditures	2,172	1,349	3,221	2,773
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	108,820	103,056	209,890	202,611
Investment income	239	56	487	101
Amortization	6,982	6,975	14,090	14,210
Depreciation	2,007	1,756	3,936	3,522
Interest expense	11,461	13,953	24,051	28,908
Income before income taxes	22,245	13,810	36,048	23,286
Total assets	2,556,293	2,516,430	2,556,293	2,516,430
Capital expenditures	1,314	1,761	3,246	3,250
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	61,287	55,417	114,701	107,245
Investment income	1	73	4	145
Amortization	2,591	2,432	5,033	4,855
Depreciation	488	561	984	1,124
Interest expense	685	216	932	445
Income before income taxes	16,287	16,390	30,849	30,874
Total assets	1,018,689	865,000	1,018,689	865,000
Capital expenditures	322	1,211	914	1,662
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	41,752	38,360	78,320	73,148
Investment income	83	1	147	1
Amortization	1,140	1,022	2,205	2,045
Depreciation	472	529	959	1,059
Interest expense	1,327	1,596	2,563	3,195
Income before income taxes	6,906	5,538	11,958	10,040
Total assets	327,957	283,996	327,957	283,996
Capital expenditures	276	301	481	541
Other
Segment Reporting Information [Line Items]
Total revenues	99	(107)	1,032	(324)
Investment income	172	109	254	190
Amortization	4	9	10	19
Depreciation	771	757	1,540	1,439
Interest expense	(13,622)	(16,656)	(28,201)	(33,746)
Income before income taxes	14,489	16,721	33,169	35,028
Total assets	(2,289,795)	(2,067,852)	(2,289,795)	(2,067,852)
Capital expenditures	$ 608	$ 248	$ 1,082	$ 371