Components of net periodic postretirement benefit cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Pension and Other Postretirement Benefits Disclosure [Line Items]
Amortization of prior service credit	$ (950)	$ 0	$ (2,850)	$ 0
Other Postretirement Benefit Plans Defined Benefit [Member]
Pension and Other Postretirement Benefits Disclosure [Line Items]
Defined Benefit Plan Service Cost	364	564	1,093	1,693
Defined Benefit Plan Interest Cost	1,712	1,712	5,135	5,136
Amortization of prior service credit	(950)	0	(2,850)	0
Defined Benefit Plan Future Amortization Of Gain Loss	0	473	0	1,419
Total net periodic pension cost	$ 1,126	$ 2,749	$ 3,378	$ 8,248