Components of net periodic postretirement benefit cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension and Other Postretirement Benefits Disclosure [Line Items]
Amortization of prior service credit	$ (950)	$ (950)	$ (1,900)	$ (1,900)
Other Postretirement Benefit Plans Defined Benefit [Member]
Pension and Other Postretirement Benefits Disclosure [Line Items]
Defined Benefit Plan Service Cost	368	364	735	729
Defined Benefit Plan Interest Cost	1,589	1,712	3,178	3,423
Amortization of prior service credit	(950)	(950)	(1,900)	(1,900)
Defined Benefit Plan Future Amortization Of Gain Loss	249	0	498	0
Total net periodic pension cost	$ 1,256	$ 1,126	$ 2,511	$ 2,252