Employee Benefits - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Maximum percentage of a participant's eligible compensation that a participating may elect to contribute, on a tax-deferred annually to the plan (as a percent) 50.00%	80.00%
Percentage of matching contribution made by company, of first 6.0% of participating employees contributions (as a percent)	50.00%
Percentage of participating employees contribution, matched 100% (as a percent)	6.00%
Employer contribution	$ 10,400	$ 9,600	$ 9,000
Non-qualified deferred gains or (losses) of investment	12,100	5,300	$ (700)
Non-qualified deferred compensation plan assets	85,589	69,912
Non-qualified deferred compensation plan liabilities	$ 85,589	$ 69,912