Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total revenues	$ 475,646	$ 462,274	$ 1,407,031	$ 1,332,965
Investment income	491	230	1,085	1,150
Amortization	21,435	21,805	64,402	65,025
Depreciation	5,489	5,195	17,242	15,867
Interest expense	9,393	9,883	28,949	29,617
Income before income taxes	124,419	116,972	343,388	328,598
Total assets	7,414,277	5,522,027	7,414,277	5,522,027	$ 5,262,734
Capital expenditures	4,049	4,191	12,897	13,135
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	234,483	228,645	712,739	695,393
Investment income	3	5	6	33
Amortization	10,540	10,861	31,704	32,743
Depreciation	1,262	1,508	3,975	4,761
Interest expense	7,216	9,026	23,918	29,415
Income before income taxes	54,950	44,894	151,783	144,496
Total assets	4,246,422	3,652,977	4,246,422	3,652,977
Capital expenditures	844	1,443	3,384	4,664
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	127,718	123,632	342,576	333,522
Investment income	124	96	279	583
Amortization	6,913	6,921	20,664	21,011
Depreciation	1,412	1,945	4,975	5,881
Interest expense	8,304	10,844	27,257	34,895
Income before income taxes	32,203	32,319	68,127	68,367
Total assets	5,026,918	2,933,568	5,026,918	2,933,568
Capital expenditures	1,357	2,153	3,885	5,399
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	71,574	70,192	208,812	184,893
Investment income	0	0	0	4
Amortization	2,845	2,882	8,621	7,915
Depreciation	471	503	1,438	1,487
Interest expense	1,515	1,540	4,803	2,472
Income before income taxes	21,219	20,862	56,569	51,711
Total assets	1,258,783	1,053,516	1,258,783	1,053,516
Capital expenditures	214	11	1,606	925
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	41,491	39,586	122,397	117,906
Investment income	72	57	224	204
Amortization	1,137	1,140	3,412	3,345
Depreciation	402	473	1,191	1,432
Interest expense	876	1,257	2,783	3,820
Income before income taxes	7,910	5,971	22,480	17,929
Total assets	432,331	342,360	432,331	342,360
Capital expenditures	364	80	856	561
Other
Segment Reporting Information [Line Items]
Total revenues	380	219	20,507	1,251
Investment income	292	72	576	326
Amortization	0	1	1	11
Depreciation	1,942	766	5,663	2,306
Interest expense	(8,518)	(12,784)	(29,812)	(40,985)
Income before income taxes	8,137	12,926	44,429	46,095
Total assets	(3,550,177)	(2,460,394)	(3,550,177)	(2,460,394)
Capital expenditures	$ 1,270	$ 504	$ 3,166	$ 1,586