Retirement and Deferred Compensation Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Contribution to defined plans	$ 1,800,000	$ 1,900,000	$ 1,900,000
Non-qualified deferred cash incentive plan maximum eligibility percentage of employees	100.00%
Mutual fund investments and deferred compensation obligation, at fair value	$ 3,200,000	1,300,000
Trading gain and compensation expense	$ 400,000	$ 200,000	$ 0