Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Plans, Defined Benefit
Components of Net Periodic Benefit Cost
Service cost	$ 6,070	$ 5,587
Interest cost	10,346	10,798
Expected return on plan assets	(11,759)	(12,195)
Amortization of prior service cost	95	69
Amortization of actuarial loss	6,420	4,882
Net periodic pension benefit cost	11,172	9,141
Pretax reclassification from AOCI included in net periodic pension benefit cost	6,515	4,951
Other Postretirement Benefit Plans, Defined Benefit
Components of Net Periodic Benefit Cost
Service cost	708	1,166
Interest cost	815	1,562
Amortization of prior service cost	(1,216)	(169)
Amortization of actuarial loss	343	287
Net periodic pension benefit cost	650	2,846
Pretax reclassification from AOCI included in net periodic pension benefit cost	$ (873)	$ 118