Components of net periodic postretirement benefit cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension and Other Postretirement Benefits Disclosure [Line Items]
Amortization of prior service cost	$ 50	$ 240	$ 150	$ 720
Other Postretirement Benefit Plans Defined Benefit [Member]
Pension and Other Postretirement Benefits Disclosure [Line Items]
Defined Benefit Plan Service Cost	548	504	1,642	1,512
Defined Benefit Plan Interest Cost	1,950	2,135	5,851	6,405
Amortization of prior service cost	(50)	(240)	(150)	(720)
Defined Benefit Plan Amortization Of Net Gains Losses	540	267	1,621	801
Defined Benefit Plan Net Periodic Benefit Cost	$ 2,988	$ 2,666	$ 8,964	$ 7,998