Business Combinations - Acquisitions Accounted for Business Combinations (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	$ 41,471	$ 124,716	$ 136,000
Other Payable	11,708	10,664	10,028
Recorded Earn-Out Payable	6,921	4,462	36,899
Net Assets Acquired	60,100	140,334	182,927
Maximum Potential Earn- Out Payable	27,451	16,285	73,074
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
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
Other Acquisitions [Member] | Various
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	41,471	26,140	12,926
Other Payable	11,708	464	95
Recorded Earn-Out Payable	6,921	400	4,584
Net Assets Acquired	60,100	27,004	17,605
Maximum Potential Earn- Out Payable	$ 27,451	7,785	$ 8,212
Various
Business Combination, Separately Recognized Transactions [Line Items]
Net Assets Acquired		$ 27,004
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
Pro Forma Weighted Average Shares Outstanding, Diluted			140,112