Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value Of Assets And Liabilities Statement [Line Items]
Other Payable	$ 5,462,000	$ 11,708,000	$ 10,664,000
Payments to Acquire Businesses, Gross	934,928,000	41,471,000	124,716,000
Recorded Earn-Out Payable	77,377,000	6,921,000	4,462,000
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets	11,054,000		2,094,000
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets		601,000	5,079,000
Fixed assets	5,829,000	69,000	684,000
Goodwill	717,710,000	42,172,000	93,376,000
Purchased customer accounts	342,008,000	18,738,000	50,625,000
Non-compete agreements	3,222,000	721,000	228,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	18,588,000		20,000
Total assets acquired	1,151,328,000	62,301,000	152,106,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	52,917,000
Other current liabilities	(32,996,000)	(1,512,000)	(8,542,000)
Deferred Tax Liabilities, Net		(689,000)	(3,230,000)
Other liabilities	(565,000)
Total liabilities assumed	(33,561,000)	(2,201,000)	(11,772,000)
Net assets acquired	1,117,767,000	60,100,000	140,334,000
Maximum Potential Earn- Out Payable	147,668,000	27,451,000	16,285,000
Social Security Advocates for the Disabled LLC [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			2,094,000
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			1,042,000
Fixed assets			307,000
Goodwill			22,352,000
Purchased customer accounts			13,069,000
Non-compete agreements			72,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			38,936,000
Other current liabilities			(1,717,000)
Deferred Tax Liabilities, Net			(3,230,000)
Total liabilities assumed			(4,947,000)
Net assets acquired			33,989,000
Various [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets	0		0
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			1,555,000
Fixed assets	100,000		77,000
Goodwill	22,712,000		19,570,000
Purchased customer accounts	15,085,000		11,075,000
Non-compete agreements	297,000		117,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	754,000		20,000
Total assets acquired	39,271,000		32,414,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	323,000
Other current liabilities	(1,715,000)		(5,410,000)
Deferred Tax Liabilities, Net			0
Other liabilities	0
Total liabilities assumed	(1,715,000)		(5,410,000)
Net assets acquired	37,556,000		27,004,000
Morstan General Agency, Inc [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			0
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			2,482,000
Fixed assets			300,000
Goodwill			51,454,000
Purchased customer accounts			26,481,000
Non-compete agreements			39,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			80,756,000
Other current liabilities			(1,415,000)
Deferred Tax Liabilities, Net			0
Total liabilities assumed			(1,415,000)
Net assets acquired			79,341,000
Other Acquisitions [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	717,700,000
Services
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	16,500,000	700,000	22,400,000
Services | Social Security Advocates for the Disabled LLC [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Other Payable			0
Payments to Acquire Businesses, Gross			32,526,000
Recorded Earn-Out Payable			971,000
Net assets acquired			33,989,000
Maximum Potential Earn- Out Payable			3,500,000
Retail
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	676,900,000	33,100,000	13,100,000
National Programs
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	18,700,000	7,200,000	57,900,000
Wholesale Brokerage
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	$ 5,500,000	$ 1,200,000	(1,200)
Wholesale Brokerage | Morstan General Agency, Inc [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Other Payable			10,200,000
Payments to Acquire Businesses, Gross			66,050,000
Recorded Earn-Out Payable			3,091,000
Net assets acquired			79,341,000
Maximum Potential Earn- Out Payable			$ 5,000,000