Pension and Other Postretirement Benefits - Amounts Recognized in Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Amounts recognized in the balance sheets:
Accrued benefit liability (noncurrent)	$ (144,320)	$ (131,237)
Pension Benefits [Member]
Amounts recognized in the balance sheets:
Prepaid benefit cost	8,728	9,501
Accrued benefit liability (current)	(3,900)	(3,896)
Accrued benefit liability (noncurrent)	(95,550)	(84,801)
Net funded status	(90,722)	(79,196)
Other Benefits [Member]
Amounts recognized in the balance sheets:
Accrued benefit liability (current)	(3,002)	(2,682)
Accrued benefit liability (noncurrent)	(48,770)	(46,436)
Net funded status	$ (51,772)	$ (49,118)