Comprehensive Income and Accumulated Other Comprehensive Income (Loss) - Summary of Effects of Reclassifications from Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Amortization of pension and other postretirement benefit plan items:
Tax benefit	$ (7,114)	$ (22,315)	$ 38,194
Total net of tax	(6,463)	22,104	(8,909)
Pension and Other Postretirement Benefit Plans [Member] | Reclassified from Accumulated Other Comprehensive Income (Loss) [Member]
Amortization of pension and other postretirement benefit plan items:
Actuarial losses	4,469
Prior service credit	(148)
Total before tax	4,321
Tax benefit	(1,664)
Total net of tax	$ 2,657