Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents		$ 25,365,000	$ 40,360,000
Payments to Acquire Businesses, Gross	$ 136,000,000	721,851,000	408,072,000
Other Payable	10,028,000	1,904,000	552,000
Recorded Earn-Out Payable	36,899,000	33,229,000	5,091,000
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets	3,640,000	20,964,000	59,598,000
Fixed assets	374,000	8,973,000	1,915,000
Goodwill	136,381,000	513,925,000	295,944,000
Purchased customer accounts	53,392,000	260,424,000	118,983,000
Non-compete agreements	328,000	1,194,000	2,886,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	14,000	20,045,000	108,000
Total assets acquired	194,129,000	1,165,141,000	519,794,000
Other current liabilities	(12,213,000)	(14,974,000)	(81,150,000)
Deferred Tax Liabilities, Net	2,576,000	(46,566,000)	(22,764,000)
Other liabilities	(1,565,000)	(32,366,000)	(2,165,000)
Total liabilities assumed	(11,202,000)	(408,157,000)	(106,079,000)
Net assets acquired	182,927,000	756,984,000	413,715,000
Maximum Potential Earn- Out Payable	73,074,000	49,462,000	16,768,000
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
Liberty Insurance Brokers, Inc. and Affiliates (Liberty)
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			393,000
Fixed assets			30,000
Goodwill			12,697,000
Purchased customer accounts			3,878,000
Non-compete agreements			31,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			17,029,000
Other current liabilities			(866,000)
Deferred Tax Liabilities, Net			0
Other liabilities			0
Total liabilities assumed			(866,000)
Net assets acquired			16,163,000
Beecher Carlson Holdings [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			40,360,000
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets			57,632,000
Fixed assets			1,786,000
Goodwill			265,174,000
Purchased customer accounts			101,565,000
Non-compete agreements			2,758,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			0
Total assets acquired			469,275,000
Other current liabilities			(80,090,000)
Deferred Tax Liabilities, Net			(22,764,000)
Other liabilities			(2,165,000)
Total liabilities assumed			(105,019,000)
Net assets acquired			364,256,000
ICA, 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			0
Fixed assets			75,000
Goodwill			12,377,000
Purchased customer accounts			7,917,000
Non-compete agreements			21,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets			107,000
Total assets acquired			20,497,000
Other current liabilities			0
Deferred Tax Liabilities, Net			0
Other liabilities			0
Total liabilities assumed			0
Net assets acquired			20,497,000
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	0
Business Acquisition Purchase Price Allocation Current Assets Prepaid Expenses And Other Assets	169,000	742,000	1,573,000
Fixed assets	59,000	1,724,000	24,000
Goodwill	21,040,000	10,417,000	5,696,000
Purchased customer accounts	(2,135,000)	4,384,000	5,623,000
Non-compete agreements	156,000	166,000	76,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0	0	1,000
Total assets acquired	19,289,000	17,433,000	12,993,000
Other current liabilities	(4,895,000)	(249,000)	(194,000)
Deferred Tax Liabilities, Net	2,576,000	0	0
Other liabilities	635,000	0	0
Total liabilities assumed	(1,684,000)	(249,000)	(194,000)
Net assets acquired	17,605,000	17,184,000	12,799,000
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
Retail
Fair Value Of Assets And Liabilities Statement [Line Items]
Goodwill	$ 113,800,000	$ 86,400,000	$ 257,200,000
Retail | Liberty Insurance Brokers, Inc. and Affiliates (Liberty)
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition			Jun. 01, 2013
Payments to Acquire Businesses, Gross			$ 13,792,000
Other Payable			50,000
Recorded Earn-Out Payable			2,321,000
Net assets acquired			16,163,000
Maximum Potential Earn- Out Payable			4,300,000
Retail | Spain Agency, Inc. (Spain)
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition	Mar. 01, 2015
Payments to Acquire Businesses, Gross	$ 20,706,000
Other Payable	0
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
Payments to Acquire Businesses, Gross	$ 9,455,000
Other Payable	0
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
Payments to Acquire Businesses, Gross	$ 49,600,000
Other Payable	400,000
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
Payments to Acquire Businesses, Gross	$ 10,142,000
Other Payable	391,000
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
Payments to Acquire Businesses, Gross	$ 68,000
Other Payable	8,442,000
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
Payments to Acquire Businesses, Gross	$ 12,096,000
Other Payable	200,000
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
Payments to Acquire Businesses, Gross	$ 12,000,000
Other Payable	0
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
Payments to Acquire Businesses, Gross		$ 90,000,000
Other Payable		0
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	$ 18,000,000	$ 420,000,000	27,100,000
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
Payments to Acquire Businesses, Gross		$ 609,183,000
Other Payable		1,471,000
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	$ 4,600,000	$ 7,700,000	$ (800,000)
Wholesale Brokerage | Axia Strategies, Inc. [Member]
Fair Value Of Assets And Liabilities Statement [Line Items]
Effective Date of Acquisition		May 01, 2014
Payments to Acquire Businesses, Gross		$ 9,870,000
Other Payable		0
Recorded Earn-Out Payable		1,824,000
Net assets acquired		11,694,000
Maximum Potential Earn- Out Payable		$ 5,200,000