Benefit Obligations - Accumulated Other Comprehensive Loss (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Accumulated other comprehensive loss, after tax	$ 622,919	$ 578,757
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net prior service cost	205	242
Net actuarial loss	577,413	527,045
Transition asset	(138)	(157)
Accumulated other comprehensive loss	577,480	527,130
Tax on prior actuarial gains	23,813	23,813
Accumulated other comprehensive loss, after tax	601,293	550,943	358,725
Post-retirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net prior service cost
Net actuarial loss	18,600	24,788
Transition asset
Accumulated other comprehensive loss	18,600	24,788
Tax on prior actuarial gains	3,026	3,026
Accumulated other comprehensive loss, after tax	$ 21,626	$ 27,814	$ 13,651