Business Combinations - Acquisitions Accounted for Business Combinations (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	$ 136,000	$ 721,851	$ 408,072
Other Payable	10,028	1,904	552
Recorded Earn-Out Payable	36,899	33,229	5,091
Net Assets Acquired	182,927	756,984	413,715
Maximum Potential Earn- Out Payable	73,074	49,462	16,768
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
Wright Insurance Group Llc [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired			$ 16,163
Wright Insurance Group Llc [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition			Jun. 01, 2013
Cash Paid			$ 13,792
Other Payable			50
Recorded Earn-Out Payable			2,321
Net Assets Acquired			16,163
Maximum Potential Earn- Out Payable			4,300
Beecher Carlson Holdings [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired			$ 364,256
Beecher Carlson Holdings [Member] | Various
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition			Jul. 01, 2013
Cash Paid			$ 364,256
Other Payable			0
Recorded Earn-Out Payable			0
Net Assets Acquired			364,256
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	12,799
Other Acquisitions [Member] | Various
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	12,926		10,254
Other Payable	95		502
Recorded Earn-Out Payable	4,584		2,043
Net Assets Acquired	17,605		12,799
Maximum Potential Earn- Out Payable	$ 8,212		7,468
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 | 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
ICA, Inc. [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired			$ 20,497
ICA, Inc. [Member] | Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Effective Date of Acquisition			Dec. 31, 2013
Cash Paid			$ 19,770
Other Payable			0
Recorded Earn-Out Payable			727
Net Assets Acquired			20,497
Maximum Potential Earn- Out Payable			$ 5,000
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