Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 619,280	$ 501,461
Investment income	1,081	601
Amortization	26,192	20,539
Depreciation	6,035	5,552
Interest Expense	15,198	9,671
Income before income taxes	148,520	118,441
Total assets	6,806,289	5,615,083	$ 6,863,968	$ 6,688,668
Capital expenditures	23,186	9,751
Operating Segments
Segment Reporting Information [Line Items]
Investment income	677
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	389,231	278,402
Investment income	0	1
Amortization	15,444	10,242
Depreciation	1,624	1,237
Interest Expense	21,153	6,797
Income before income taxes	93,841	70,399
Total assets	5,773,290	4,687,456
Capital expenditures	3,136	2,407
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	109,560	112,475
Investment income	316	114
Amortization	6,596	6,323
Depreciation	1,590	1,387
Interest Expense	5,354	7,496
Income before income taxes	19,316	20,778
Total assets	2,852,824	3,009,147
Capital expenditures	4,568	2,678
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	70,509	66,148
Investment income	40	0
Amortization	2,849	2,837
Depreciation	404	427
Interest Expense	1,327	1,435
Income before income taxes	15,569	11,383
Total assets	1,292,406	1,323,046
Capital expenditures	1,189	420
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	49,492	44,066
Investment income	48	73
Amortization	1,303	1,137
Depreciation	284	411
Interest Expense	872	594
Income before income taxes	9,177	8,816
Total assets	480,108	432,920
Capital expenditures	109	285
Other
Segment Reporting Information [Line Items]
Total revenues	488	370
Investment income	677	413
Amortization	0	0
Depreciation	2,133	2,090
Interest Expense	(13,508)	(6,651)
Income before income taxes	10,617	7,065
Total assets	(3,592,339)	(3,837,486)
Capital expenditures	$ 14,184	$ 3,961