Pension and Postretirement Benefits - Changes in Level Three Fixed Income Instrument Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of plan assets at beginning of year	$ 593,927
Fair value of plan assets at end of year	532,403	$ 593,927
Fixed Income [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of plan assets at beginning of year	471,704
Fair value of plan assets at end of year	421,987	471,704
Level 3 [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of plan assets at beginning of year	5,013
Fair value of plan assets at end of year	5,752	5,013
Level 3 [Member] | Fixed Income [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value of plan assets at beginning of year	5,013	3,769
Purchases, net	1,269	1,347
Total unrealized loss	(530)	(103)
Fair value of plan assets at end of year	$ 5,752	$ 5,013