Components of net periodic postretirement benefit cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pension and Other Postretirement Benefits Disclosure [Line Items]
Amortization of prior service credit	$ 0	$ (50)	$ 0	$ (100)
Other Postretirement Benefit Plans Defined Benefit [Member]
Pension and Other Postretirement Benefits Disclosure [Line Items]
Defined Benefit Plan Service Cost	564	548	1,128	1,096
Defined Benefit Plan Interest Cost	1,712	1,950	3,424	3,900
Amortization of prior service credit	0	(50)	0	(100)
Defined Benefit Plan Amortization Of Net Gains Losses	473	540	946	1,080
Total net periodic pension cost	$ 2,749	$ 2,988	$ 5,498	$ 5,976