Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total revenues	$ 501,461	$ 465,080
Investment income	601	243
Amortization	20,539	21,620
Depreciation	5,552	6,098
Interest expense	9,671	9,682
Income before income taxes	118,441	110,967
Total assets	5,615,083	5,197,444	$ 5,747,550
Capital expenditures	9,751	3,032
Operating Segments
Segment Reporting Information [Line Items]
Investment income	413
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	278,402	239,287
Investment income	1	2
Amortization	10,242	10,647
Depreciation	1,237	1,389
Interest expense	6,797	8,576
Income before income taxes	70,399	48,855
Total assets	4,687,456	3,861,118
Capital expenditures	2,407	1,136
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	112,475	101,183
Investment income	114	74
Amortization	6,323	6,904
Depreciation	1,387	1,959
Interest expense	7,496	10,035
Income before income taxes	20,778	12,527
Total assets	3,009,147	2,634,008
Capital expenditures	2,678	1,080
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	66,148	65,247
Investment income	0	0
Amortization	2,837	2,931
Depreciation	427	490
Interest expense	1,435	1,675
Income before income taxes	11,383	15,265
Total assets	1,323,046	1,121,109
Capital expenditures	420	378
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	44,066	39,326
Investment income	73	80
Amortization	1,137	1,138
Depreciation	411	399
Interest expense	594	961
Income before income taxes	8,816	6,121
Total assets	432,920	363,173
Capital expenditures	285	150
Other
Segment Reporting Information [Line Items]
Total revenues	370	20,037
Investment income	413	87
Amortization	0	0
Depreciation	2,090	1,861
Interest expense	(6,651)	(11,565)
Income before income taxes	7,065	28,199
Total assets	(3,837,486)	(2,781,964)
Capital expenditures	$ 3,961	$ 288