Components of net periodic postretirement benefit cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Pension and Other Postretirement Benefits Disclosure [Line Items]
Amortization of prior service credit	$ (50)	$ (240)
Other Postretirement Benefit Plans Defined Benefit [Member]
Pension and Other Postretirement Benefits Disclosure [Line Items]
Defined Benefit Plan Service Cost	548	504
Defined Benefit Plan Interest Cost	1,950	2,136
Amortization of prior service credit	(50)	(240)
Defined Benefit Plan Amortization Of Net Gains Losses	540	267
Defined Benefit Plan Net Periodic Benefit Cost	$ 2,988	$ 2,667