Components of Post-Retirement Benefit Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,690	$ 2,339	$ 5,381	$ 4,678
Interest cost	4,145	4,025	8,297	8,051
Expected return on assets	(4,117)	(3,896)	(8,237)	(7,926)
Prior service cost	515	518	1,031	1,037
Net actuarial (gain)/loss	2,992	2,395	5,958	4,790
Net periodic benefit cost	6,225	5,381	12,430	10,630
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	120	218	244	466
Interest cost	256	250	513	502
Expected return on assets	0	0	0	0
Prior service cost	0	0	0	0
Net actuarial (gain)/loss	0	(268)	0	(404)
Net periodic benefit cost	$ 376	$ 200	$ 757	$ 564