Pension and Post-Retirement Plans (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Components of net periodic (credit) expense:
Interest cost	[1]	$ 1,271	$ 1,404	$ 2,543	$ 2,808
Expected return on plan assets	[1]	(4,035)	(4,073)	(8,071)	(8,146)
Amortization of net actuarial loss	[1]	1,795	2,053	3,591	4,106
Net periodic (credit) expense	[1]	(969)	(616)	(1,937)	(1,232)
Post-Retirement Benefits
Components of net periodic (credit) expense:
Interest cost	[1]	128	144	256	288
Amortization of prior-service costs	[1]	(62)	(62)	(124)	(124)
Amortization of net actuarial loss	[1]	76	68	153	136
Net periodic (credit) expense	[1]	$ 142	$ 150	$ 285	$ 300

[1] Amounts are included in G&A expense on the Consolidated Statements of Income and Comprehensive Income.