Segment Information - Summarized Financial Information Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 575,219	$ 473,187	$ 1,194,499	$ 974,648
Investment income	1,525	731	2,606	1,332
Amortization	25,954	20,785	52,146	41,324
Depreciation	5,666	5,599	11,701	11,151
Interest Expense	16,293	10,052	31,491	19,723
Income before income taxes	123,522	100,910	272,042	219,351
Total assets	7,164,767	5,701,557	7,164,767	5,701,557	$ 7,164,767	$ 6,863,968	$ 6,688,668
Capital expenditures	11,988	9,639	35,174	19,390
Operating Segments
Segment Reporting Information [Line Items]
Investment income	1,051		1,728
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	310,738	233,317	699,969
Investment income	17	0	17
Amortization	15,357	10,502
Depreciation	1,919	1,273
Interest Expense	21,201	7,112
Income before income taxes	44,217	44,361
Total assets		4,668,251		4,668,251	5,925,156
Capital expenditures	2,408	1,954
Operating Segments | National Programs
Segment Reporting Information [Line Items]
Total revenues	131,817	118,440	241,377
Investment income	380	132	696
Amortization	6,357	6,324
Depreciation	1,621	1,354
Interest Expense	4,223	6,376
Income before income taxes	40,243	24,324
Total assets		2,844,485		2,844,485	2,981,933
Capital expenditures	1,942	2,215
Operating Segments | Wholesale Brokerage
Segment Reporting Information [Line Items]
Total revenues	81,171	75,568	151,680
Investment income	43	81	83
Amortization	2,844	2,822
Depreciation	425	426
Interest Expense	1,260	1,371
Income before income taxes	22,944	20,547
Total assets		1,267,207		1,267,207	1,352,659
Capital expenditures	513	447
Operating Segments | Services
Segment Reporting Information [Line Items]
Total revenues	50,797	45,867	100,289
Investment income	34	36	82
Amortization	1,396	1,137
Depreciation	303	379
Interest Expense	1,203	583
Income before income taxes	9,268	8,085
Total assets		419,876		419,876	466,162
Capital expenditures	222	147
Other
Segment Reporting Information [Line Items]
Total revenues	696	(5)	$ 1,184
Investment income	1,051	482
Amortization	0	0
Depreciation	1,398	2,167
Interest Expense	(11,594)	(5,390)
Income before income taxes	6,850	3,593
Total assets		(3,498,262)		$ (3,498,262)	$ (3,561,143)
Capital expenditures	$ 6,903	$ 4,876