Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Of Assets And Liabilities Statement [Line Items]
Other Payable	$ 11,708,000	$ 10,664,000	$ 10,028,000
Payments to Acquire Businesses, Gross	41,471,000	124,716,000	136,000,000.0
Recorded Earn-Out Payable	6,921,000	4,462,000	36,899,000
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	601,000	5,079,000	3,640,000
Fixed assets	69,000	684,000	374,000
Goodwill	42,172,000	93,376,000	136,381,000
Purchased customer accounts	18,738,000	50,625,000	53,392,000
Non-compete agreements	721,000	228,000	328,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets		20,000	14,000
Total assets acquired	62,301,000	152,106,000	194,129,000
Other current liabilities	(1,512,000)	(8,542,000)	(12,213,000)
Deferred Tax Liabilities, Net	(689,000)	(3,230,000)
Deferred Tax Assets, Net			2,576,000
Other liabilities			(1,565,000)
Total liabilities assumed	(2,201,000)	(11,772,000)	(11,202,000)
Net assets acquired	60,100,000	140,334,000	182,927,000
Maximum Potential Earn- Out Payable	27,451,000	16,285,000	73,074,000
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
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets		1,555,000
Fixed assets		77,000
Goodwill		19,570,000
Purchased customer accounts		11,075,000
Non-compete agreements		117,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets		20,000
Total assets acquired		32,414,000
Other current liabilities		(5,410,000)
Deferred Tax Liabilities, Net		0
Total liabilities assumed		(5,410,000)
Net assets acquired		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
Spain Agency, Inc. (Spain)
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			324,000
Fixed assets			50,000
Goodwill			15,748,000
Purchased customer accounts			7,430,000
Non-compete agreements			21,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			23,573,000
Other current liabilities			(250,000)
Deferred Tax Liabilities, Net			0
Other liabilities			0
Total liabilities assumed			(250,000)
Net assets acquired			23,323,000
Bellingham Underwriters, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			0
Fixed assets			25,000
Goodwill			9,608,000
Purchased customer accounts			3,223,000
Non-compete agreements			21,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			12,877,000
Other current liabilities			(48,000)
Deferred Tax Liabilities, Net			0
Other liabilities			0
Total liabilities assumed			(48,000)
Net assets acquired			12,829,000
Fitness Insurance, LLC [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			9,000
Fixed assets			17,000
Goodwill			8,105,000
Purchased customer accounts			3,715,000
Non-compete agreements			0
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			11,846,000
Other current liabilities			(12,000)
Deferred Tax Liabilities, Net			0
Other liabilities			0
Total liabilities assumed			(12,000)
Net assets acquired			11,834,000
Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			652,000
Fixed assets			41,000
Goodwill			39,859,000
Purchased customer accounts			23,000,000
Non-compete agreements			21,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			14,000
Total assets acquired			63,587,000
Other current liabilities			0
Deferred Tax Liabilities, Net			0
Other liabilities			0
Total liabilities assumed			0
Net assets acquired			63,587,000
Bentrust Financial, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			0
Fixed assets			36,000
Goodwill			8,166,000
Purchased customer accounts			2,789,000
Non-compete agreements			43,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			11,034,000
Other current liabilities			(182,000)
Deferred Tax Liabilities, Net			0
Other liabilities			0
Total liabilities assumed			(182,000)
Net assets acquired			10,852,000
MBA Insurance Agency of Arizona, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			0
Fixed assets			33,000
Goodwill			13,471,000
Purchased customer accounts			7,338,000
Non-compete agreements			11,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			20,853,000
Other current liabilities			(6,280,000)
Deferred Tax Liabilities, Net			0
Other liabilities			0
Total liabilities assumed			(6,280,000)
Net assets acquired			14,573,000
Smith Insurance, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			0
Fixed assets			73,000
Goodwill			10,374,000
Purchased customer accounts			3,526,000
Non-compete agreements			31,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets			14,004,000
Other current liabilities			(504,000)
Deferred Tax Liabilities, Net			0
Other liabilities			(157,000)
Total liabilities assumed			(661,000)
Net assets acquired			13,343,000
Other Acquisitions [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			169,000
Fixed assets			59,000
Goodwill			21,040,000
Purchased customer accounts			(2,135,000)
Non-compete agreements			156,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			19,289,000
Other current liabilities			(4,895,000)
Deferred Tax Assets, Net			2,576,000
Other liabilities			635,000
Total liabilities assumed			(1,684,000)
Net assets acquired			17,605,000
Liberty Insurance Brokers, Inc. and Affiliates [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			2,486,000
Fixed assets			40,000
Goodwill			10,010,000
Purchased customer accounts			4,506,000
Non-compete agreements			24,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			17,066,000
Other current liabilities			(42,000)
Deferred Tax Liabilities, Net			0
Other liabilities			(2,043,000)
Total liabilities assumed			(2,085,000)
Net assets acquired			14,981,000
Services
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	700,000	$ 22,400,000
Services | Social Security Advocates for the Disabled LLC [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition		Feb. 01, 2016
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	33,100,000	13,100,000	$ 113,800,000
Retail | Spain Agency, Inc. (Spain)
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition			Mar. 01, 2015
Other Payable			$ 0
Payments to Acquire Businesses, Gross			20,706,000
Recorded Earn-Out Payable			2,617,000
Net assets acquired			23,323,000
Maximum Potential Earn- Out Payable			$ 9,162,000
Retail | Fitness Insurance, LLC [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition			Jun. 01, 2015
Other Payable			$ 0
Payments to Acquire Businesses, Gross			9,455,000
Recorded Earn-Out Payable			2,379,000
Net assets acquired			11,834,000
Maximum Potential Earn- Out Payable			$ 3,500,000
Retail | Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition			Jun. 01, 2015
Other Payable			$ 400,000
Payments to Acquire Businesses, Gross			49,600,000
Recorded Earn-Out Payable			13,587,000
Net assets acquired			63,587,000
Maximum Potential Earn- Out Payable			$ 26,000,000
Retail | Bentrust Financial, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition			Dec. 01, 2015
Other Payable			$ 391,000
Payments to Acquire Businesses, Gross			10,142,000
Recorded Earn-Out Payable			319,000
Net assets acquired			10,852,000
Maximum Potential Earn- Out Payable			$ 2,200,000
Retail | MBA Insurance Agency of Arizona, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition			Dec. 01, 2015
Other Payable			$ 8,442,000
Payments to Acquire Businesses, Gross			68,000
Recorded Earn-Out Payable			6,063,000
Net assets acquired			14,573,000
Maximum Potential Earn- Out Payable			$ 9,500,000
Retail | Smith Insurance, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition			Dec. 01, 2015
Other Payable			$ 200,000
Payments to Acquire Businesses, Gross			12,096,000
Recorded Earn-Out Payable			1,047,000
Net assets acquired			13,343,000
Maximum Potential Earn- Out Payable			$ 6,350,000
Retail | Liberty Insurance Brokers, Inc. and Affiliates [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition			Feb. 01, 2015
Other Payable			$ 0
Payments to Acquire Businesses, Gross			12,000,000
Recorded Earn-Out Payable			2,981,000
Net assets acquired			14,981,000
Maximum Potential Earn- Out Payable			3,750,000
National Programs
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	7,200,000	57,900,000	$ 18,000,000.0
National Programs | Bellingham Underwriters, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition			May 01, 2015
Wholesale Brokerage
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	$ 1,200,000	$ (1,200)	$ 4,600,000
Wholesale Brokerage | Morstan General Agency, Inc [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition		Jun. 01, 2016
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