Pensions - Reconciliation for Level 3 Inputs Used in Determining Fair Value (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance of assets classified as Level 3	$ 16,454	$ 4,709
Purchases, sales and settlements, net	(16,348)	(4,380)
Total gain	4	12
Transfers into (out of) Level 3	0
Ending balance of assets classified as Level 3	$ 110	16,454
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		$ 16,113