Pension Plans and Other Postretirement Benefits - Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss (gain)	$ 343,374	$ (260,452)
Prior service cost	4,619
Amortization of prior service credit	7,911	7,915	6,538
Amortization of loss	(14,494)	(41,436)	(37,916)
Total recognized in other comprehensive income, pre-tax	341,410	(293,973)
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss (gain)	335,581	(247,622)
Prior service cost	4,619
Amortization of prior service credit	6,552	6,556
Amortization of loss	(14,494)	(41,076)
Total recognized in other comprehensive income, pre-tax	332,258	(282,142)
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss (gain)	7,793	(12,830)
Amortization of prior service credit	1,359	1,359
Amortization of loss		(360)
Total recognized in other comprehensive income, pre-tax	$ 9,152	$ (11,831)