Pension and Post-Retirement Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits
Components of net periodic (credit) expense:
Interest cost	$ 1,404	$ 1,470	$ 4,211	$ 4,411
Expected return on plan assets	(4,073)	(3,906)	(12,217)	(11,720)
Amortization of net actuarial loss	2,053	2,262	6,157	6,786
Net periodic (credit) expense	(616)	(174)	(1,849)	(523)
Post-Retirement Benefits
Components of net periodic (credit) expense:
Interest cost	144	160	433	479
Service cost		1		3
Amortization of prior-service costs	(62)	(62)	(187)	(187)
Amortization of net actuarial loss	68	81	206	245
Net periodic (credit) expense	$ 150	$ 180	$ 452	$ 540