Benefit Obligations - Components of Net Periodic Benefit Cost or Credit (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 415	$ 531	$ 472
Interest cost	61,456	67,499	70,337
Expected return on plan assets	(68,076)	(67,965)	(71,464)
Amortization of net actuarial loss, prior service cost and transition asset	14,360	6,768	2,193
Curtailments	326	54
Total benefit cost/(credit)	8,481	6,887	1,538
Post-retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	89	75	81
Interest cost	2,600	2,597	2,441
Expected return on plan assets	(3,811)	(4,128)	(4,597)
Amortization of net actuarial loss, prior service cost and transition asset	2,017	552
Total benefit cost/(credit)	$ 895	$ (904)	$ (2,075)