Employer Sponsored Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Components of net periodic benefit cost
Interest cost	$ 514	$ 535	$ 1,543	$ 1,605
Expected return on assets	(594)	(605)	(1,782)	(1,815)
Amortization of actuarial loss	273	233	819	700
Net periodic benefit cost	$ 193	$ 163	$ 580	$ 490