Business Combinations - Acquisitions Accounted for Business Combinations (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	$ 124,716	$ 136,000	$ 721,851
Other Payable	10,664	10,028	1,904
Recorded Earn-Out Payable	4,462	36,899	33,229
Net Assets Acquired	140,334	182,927	756,984
Maximum Potential Earn- Out Payable	16,285	73,074	49,462
Liberty Insurance Brokers, Inc. and Affiliates [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired		$ 14,981
Liberty Insurance Brokers, Inc. and Affiliates [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition		Feb. 01, 2015
Cash Paid		$ 12,000
Other Payable		0
Recorded Earn-Out Payable		2,981
Net Assets Acquired		14,981
Maximum Potential Earn- Out Payable		3,750
Wright Insurance Group Llc [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired			$ 610,654
Wright Insurance Group Llc [Member] | National Programs
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition			May 01, 2014
Cash Paid			$ 609,183
Other Payable			1,471
Recorded Earn-Out Payable			0
Net Assets Acquired			610,654
Maximum Potential Earn- Out Payable			0
Spain Agency, Inc. (Spain)
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired		$ 23,323
Spain Agency, Inc. (Spain) | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition		Mar. 01, 2015
Cash Paid		$ 20,706
Other Payable		0
Recorded Earn-Out Payable		2,617
Net Assets Acquired		23,323
Maximum Potential Earn- Out Payable		9,162
Bellingham Underwriters, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired		$ 12,829
Bellingham Underwriters, Inc. [Member] | National Programs
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition		May 01, 2015
Bellingham Underwriters, Inc. [Member] | National Programs Divisions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid		$ 9,007
Other Payable		500
Recorded Earn-Out Payable		3,322
Net Assets Acquired		12,829
Maximum Potential Earn- Out Payable		4,400
Fitness Insurance, LLC [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired		$ 11,834
Fitness Insurance, LLC [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition		Jun. 01, 2015
Cash Paid		$ 9,455
Other Payable		0
Recorded Earn-Out Payable		2,379
Net Assets Acquired		11,834
Maximum Potential Earn- Out Payable		3,500
Other
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired		$ 63,587
Other | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition		Jun. 01, 2015
Cash Paid		$ 49,600
Other Payable		400
Recorded Earn-Out Payable		13,587
Net Assets Acquired		63,587
Maximum Potential Earn- Out Payable		26,000
Bentrust Financial, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired		$ 10,852
Bentrust Financial, Inc. [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition		Dec. 01, 2015
Cash Paid		$ 10,142
Other Payable		391
Recorded Earn-Out Payable		319
Net Assets Acquired		10,852
Maximum Potential Earn- Out Payable		2,200
MBA Insurance Agency of Arizona, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired		$ 14,573
MBA Insurance Agency of Arizona, Inc. [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition		Dec. 01, 2015
Cash Paid		$ 68
Other Payable		8,442
Recorded Earn-Out Payable		6,063
Net Assets Acquired		14,573
Maximum Potential Earn- Out Payable		9,500
Smith Insurance, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired		$ 13,343
Smith Insurance, Inc. [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition		Dec. 01, 2015
Cash Paid		$ 12,096
Other Payable		200
Recorded Earn-Out Payable		1,047
Net Assets Acquired		13,343
Maximum Potential Earn- Out Payable		6,350
Other Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired		17,605	17,184
Other Acquisitions [Member] | Various
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	26,140	12,926
Other Payable	464	95
Recorded Earn-Out Payable	400	4,584
Net Assets Acquired	27,004	17,605
Maximum Potential Earn- Out Payable	7,785	$ 8,212
Pacific Resources Benefits Advisors [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired			$ 117,452
Pacific Resources Benefits Advisors [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition			May 01, 2014
Cash Paid			$ 90,000
Other Payable			0
Recorded Earn-Out Payable			27,452
Net Assets Acquired			117,452
Maximum Potential Earn- Out Payable			35,000
Axia Strategies, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired			$ 11,694
Axia Strategies, Inc. [Member] | Wholesale Brokerage [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition			May 01, 2014
Cash Paid			$ 9,870
Other Payable			0
Recorded Earn-Out Payable			1,824
Net Assets Acquired			11,694
Maximum Potential Earn- Out Payable			5,200
Various
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired	$ 27,004
Various | Other Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid			12,798
Other Payable			433
Recorded Earn-Out Payable			3,953
Net Assets Acquired			17,184
Maximum Potential Earn- Out Payable			$ 9,262
Two Thousand Fourteen Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted			$ 1.49	$ 1.70
Pro Forma Weighted Average Shares Outstanding, Diluted			142,891	142,624
Two Thousand Fifteen Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted		$ 1.73	$ 1.48
Pro Forma Weighted Average Shares Outstanding, Diluted		140,112	142,891