Employer Sponsored Benefit Plan - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Interest cost	$ 2,058	$ 2,139	$ 2,066
Expected return on plan assets	(2,376)	(2,419)	(2,360)
Amortization of actuarial net loss	1,092	933	897
Total net periodic benefit cost	$ 774	$ 653	$ 603