Employee Benefit Plans - Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plans [Member]
Components of Net Periodic Benefit Cost (Credit):
Service cost	$ 335	$ 314	$ 675	$ 621
Interest cost	4,086	5,246	8,067	10,459
Expected return on plan assets	(6,455)	(7,233)	(12,560)	(14,399)
Amortization of:
Prior service cost		604	30	1,194
Net actuarial loss (gain)	1,552	1,012	2,884	2,001
Net periodic benefit cost (credit)	$ (482)	(57)	$ (904)	(124)
Medical Plan [Member]
Components of Net Periodic Benefit Cost (Credit):
Service cost		2		6
Interest cost		38		83
Amortization of:
Net actuarial loss (gain)		(92)		(92)
Net periodic benefit cost (credit)		$ (52)		$ (3)