Components of Net Periodic Benefit Expense (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 Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7,094	$ 6,035	$ 14,697	$ 11,923
Interest cost	7,700	7,545	14,131	14,837
Expected return on plan assets	(7,569)	(6,520)	(13,269)	(12,825)
Amortization of prior service cost	303	313	579	625
Amortization of transitional asset	121	116	241	227
Recognized actuarial loss	4,759	3,847	8,291	7,614
Special termination benefits	0	6,170	0	6,170
Net periodic benefit expense	12,408	17,506	24,670	28,571
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,230	1,049	2,397	2,090
Interest cost	1,279	1,342	2,513	2,791
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	(44)	(43)	(86)	(89)
Amortization of transitional asset	2	2	4	4
Recognized actuarial loss	473	452	930	941
Special termination benefits	0	0	0	0
Net periodic benefit expense	$ 2,940	$ 2,802	$ 5,758	$ 5,737