Cumulative Other Comprehensive Loss - Cumulative Other Comprehensive Loss in Accompanying Consolidated Statements of Equity (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount of gain recognized in other comprehensive loss, tax portion	$ (1,011)	$ 2,174	$ 670
Amount of gain reclassified from Cumulative Other Comprehensive Loss	13,446	3,699
Income tax benefit (expense) on derivative instruments	5,167	1,220
Other comprehensive (gain) loss, pre-tax	30,476	157,087
Other comprehensive loss, tax	7,213	60,350
Amortization of prior service credit	566	566	566
Amortization of actuarial loss	(46,736)	(36,473)
Income tax provision on postretirement benefit plans	23,410	(50,317)	84,713
Derivatives Designated as Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives (Effective Portion)	11,127	9,020	$ 2,943
Amount of gain recognized in other comprehensive loss, tax portion	$ 4,156	$ 3,394