Acquisitions (First Half 2017) - Additional Information (Detail) $ in Thousands	6 Months Ended		9 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($) Business	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)
Business Acquisition Contingent Consideration [Line Items]
Number of businesses acquired | Business		3
Consideration paid in cash	$ 23,400	$ 23,700
Consideration paid in common stock	900	2,000
Contingent consideration	12,400	$ 17,500	$ 12,400		$ 17,500
Contingent consideration, current			21,136	$ 15,151	5,900
Contingent consideration, non-current			18,760	$ 22,423	$ 11,600
Annual revenue	$ 9,100		$ 23,200
C M F Associates Limited Liability Company
Business Acquisition Contingent Consideration [Line Items]
Acquired entity, name		CMF Associates, LLC
Effective date of acquisition		Jun. 01, 2017
Slaton Insurance
Business Acquisition Contingent Consideration [Line Items]
Acquired entity, name		Slaton Insurance
Effective date of acquisition		Jun. 01, 2017
Pacific Coastal Pension And Insurance Services Inc
Business Acquisition Contingent Consideration [Line Items]
Acquired entity, name		Pacific Coastal Pension and Insurance Services, Inc.
Effective date of acquisition		Feb. 01, 2017