Employee Retirement and Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Plans
Components of net periodic benefit cost:
Service cost	$ 752	$ 793	$ 1,504	$ 1,586
Interest cost	2,927	3,043	5,854	6,086
Expected return on plan assets	(4,758)	(4,633)	(9,516)	(9,266)
Amortizations:
Prior service cost	176	214	352	428
Unrecognized net (gain) loss	2,581	2,206	5,162	4,412
Net periodic benefit cost	1,678	1,623	3,356	3,246
Postretirement Benefits
Components of net periodic benefit cost:
Service cost	146	160	292	320
Interest cost	240	271	480	542
Amortizations:
Prior service cost	23	23	46	46
Unrecognized net (gain) loss	(114)	(61)	(228)	(122)
Net periodic benefit cost	$ 295	$ 393	$ 590	$ 786