Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			$ 25,365,000
Other Payable	$ 10,664,000	$ 10,028,000	1,904,000
Payments to Acquire Businesses, Gross	124,716,000	136,000,000	721,851,000
Recorded Earn-Out Payable	4,462,000	36,899,000	33,229,000
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	5,079,000	3,640,000	20,964,000
Fixed assets	684,000	374,000	8,973,000
Goodwill	93,376,000	136,381,000	513,925,000
Purchased customer accounts	50,625,000	53,392,000	260,424,000
Non-compete agreements	228,000	328,000	1,194,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	20,000	14,000	20,045,000
Total assets acquired	152,106,000	194,129,000	1,165,141,000
Other current liabilities	(8,542,000)	(12,213,000)	(14,974,000)
Deferred Tax Liabilities, Net	(3,230,000)	2,576,000	(46,566,000)
Other liabilities		(1,565,000)	(32,366,000)
Total liabilities assumed	(11,772,000)	(11,202,000)	(408,157,000)
Net assets acquired	140,334,000	182,927,000	756,984,000
Maximum Potential Earn- Out Payable	16,285,000	73,074,000	49,462,000
Business Acquisitions Purchase Price Allocation Losses And Loss Adjustment Expense			(25,238,000)
Business Acquisition Purchase Price Allocation Unearned Premium			(289,013,000)
Business Acquisition Purchase Price Allocation Reinsurance Recoverables			25,238,000
Business Acquisition Purchase Price Allocation Prepaid Reinsurance Premiums			289,013,000
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 Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			0
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets		169,000	742,000
Fixed assets		59,000	1,724,000
Goodwill		21,040,000	10,417,000
Purchased customer accounts		(2,135,000)	4,384,000
Non-compete agreements		156,000	166,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets		0	0
Total assets acquired		19,289,000	17,433,000
Other current liabilities		(4,895,000)	(249,000)
Deferred Tax Liabilities, Net		2,576,000	0
Other liabilities		635,000	0
Total liabilities assumed		(1,684,000)	(249,000)
Net assets acquired		17,605,000	17,184,000
Business Acquisitions Purchase Price Allocation Losses And Loss Adjustment Expense			0
Business Acquisition Purchase Price Allocation Unearned Premium			0
Business Acquisition Purchase Price Allocation Reinsurance Recoverables			0
Business Acquisition Purchase Price Allocation Prepaid Reinsurance Premiums			0
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
Wright Insurance Group Llc [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			25,365,000
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			16,474,000
Fixed assets			7,172,000
Goodwill			420,209,000
Purchased customer accounts			213,677,000
Non-compete agreements			966,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			20,045,000
Total assets acquired			1,018,159,000
Other current liabilities			(14,322,000)
Deferred Tax Liabilities, Net			(46,566,000)
Other liabilities			(32,366,000)
Total liabilities assumed			(407,505,000)
Net assets acquired			610,654,000
Business Acquisitions Purchase Price Allocation Losses And Loss Adjustment Expense			(25,238,000)
Business Acquisition Purchase Price Allocation Unearned Premium			(289,013,000)
Business Acquisition Purchase Price Allocation Reinsurance Recoverables			25,238,000
Business Acquisition Purchase Price Allocation Prepaid Reinsurance Premiums			289,013,000
Pacific Resources Benefits Advisors [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			0
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			3,647,000
Fixed assets			53,000
Goodwill			76,023,000
Purchased customer accounts			38,111,000
Non-compete agreements			21,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			117,855,000
Other current liabilities			(403,000)
Deferred Tax Liabilities, Net			0
Other liabilities			0
Total liabilities assumed			(403,000)
Net assets acquired			117,452,000
Business Acquisitions Purchase Price Allocation Losses And Loss Adjustment Expense			0
Business Acquisition Purchase Price Allocation Unearned Premium			0
Business Acquisition Purchase Price Allocation Reinsurance Recoverables			0
Business Acquisition Purchase Price Allocation Prepaid Reinsurance Premiums			0
Axia Strategies, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			0
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			101,000
Fixed assets			24,000
Goodwill			7,276,000
Purchased customer accounts			4,252,000
Non-compete agreements			41,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			11,694,000
Other current liabilities			0
Deferred Tax Liabilities, Net			0
Other liabilities			0
Total liabilities assumed			0
Net assets acquired			11,694,000
Business Acquisitions Purchase Price Allocation Losses And Loss Adjustment Expense			0
Business Acquisition Purchase Price Allocation Unearned Premium			0
Business Acquisition Purchase Price Allocation Reinsurance Recoverables			0
Business Acquisition Purchase Price Allocation Prepaid Reinsurance Premiums			0
Services
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	$ 22,400,000		(200,000)
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	13,100,000	$ 113,800,000	$ 86,400,000
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
Retail | Pacific Resources Benefits Advisors [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition			May 01, 2014
Other Payable			$ 0
Payments to Acquire Businesses, Gross			90,000,000
Recorded Earn-Out Payable			27,452,000
Net assets acquired			117,452,000
Maximum Potential Earn- Out Payable			35,000,000
National Programs
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	57,900,000	$ 18,000,000	$ 420,000,000
National Programs | Bellingham Underwriters, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition		May 01, 2015
National Programs | Wright Insurance Group Llc [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition			May 01, 2014
Other Payable			$ 1,471,000
Payments to Acquire Businesses, Gross			609,183,000
Recorded Earn-Out Payable			0
Net assets acquired			610,654,000
Maximum Potential Earn- Out Payable			0
Wholesale Brokerage
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	$ 0	$ 4,600,000	$ 7,700,000
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
Wholesale Brokerage | Axia Strategies, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition			May 01, 2014
Other Payable			$ 0
Payments to Acquire Businesses, Gross			9,870,000
Recorded Earn-Out Payable			1,824,000
Net assets acquired			11,694,000
Maximum Potential Earn- Out Payable			$ 5,200,000