Pension Plans and other Postretirement Benefits - Amortization of Amounts in Accumulated Other Comprehensive Income Expected to be Recognized as Components of Net Periodic Benefit Expense (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Amortization of net prior service cost (credit)	$ 557
Amortization of net loss (gain)	44,416
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Amortization of net prior service cost (credit)	(4,729)
Amortization of net loss (gain)	$ (830)