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) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Amortization of net prior service credit	$ (6,005)
Amortization of net loss	44,653
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Amortization of net prior service credit	(1,359)
Amortization of net loss	$ 91