Benefit Obligations - Changes in Accumulated Other Comprehensive Loss Related to Pension and Post-Retirement Plans (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Accumulated other comprehensive loss at January 1, after tax	$ 578,757
Current year actuarial loss/(gain)	60,539	213,701	42,368
Accumulated other comprehensive loss at December 31, after tax	622,919	578,757
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Accumulated other comprehensive loss at January 1, after tax	550,943	358,725
Amortization of prior service cost and transition asset	(18)	(18)
Amortization of actuarial loss	(14,342)	(6,750)
Current year actuarial loss/(gain)	64,710	198,986
Accumulated other comprehensive loss at December 31, after tax	601,293	550,943
Post-retirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Accumulated other comprehensive loss at January 1, after tax	27,814	13,651
Amortization of prior service cost and transition asset
Amortization of actuarial loss	(2,017)	(552)
Current year actuarial loss/(gain)	(4,171)	14,715
Accumulated other comprehensive loss at December 31, after tax	$ 21,626	$ 27,814