Pension Plans and Other Postretirement Benefits - Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss	$ 24,200	$ 343,374
Prior service cost		4,619
Amortization of prior service credit	7,364	7,911	$ 7,915
Amortization of loss	(44,931)	(14,494)
Total recognized in other comprehensive income, pre-tax	(13,367)	341,410
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss	26,631	335,581
Prior service cost		4,619
Amortization of prior service credit	6,005	6,552
Amortization of loss	(44,825)	(14,494)
Total recognized in other comprehensive income, pre-tax	(12,189)	332,258
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss	(2,431)	7,793
Amortization of prior service credit	1,359	1,359
Amortization of loss	(106)
Total recognized in other comprehensive income, pre-tax	$ (1,178)	$ 9,152