Components of Net Periodic Benefit Cost, Amounts Recognized in Other Comprehensive Income, Weighted-Average Assumptions and Assumed Trend Rates of Plans (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income
Reclassification of actuarial loss to net periodic postretirement benefit cost	$ (11,965)	$ (7,710)	$ (3,590)
Reclassification of prior service credit to net periodic postretirement benefit cost	(11,965)	(7,710)	(3,590)
Assumed Healthcare Cost Trend Rates at December 31
Healthcare cost trend rate assumed for next year	8.00%
Rate to which the cost trend rate gradually declines	5.00%
Year that the rate reaches the rate it is assumed to maintain	2019
Other Postretirement Benefit Plans, Defined Benefit
Components of Net Periodic Postretirement Benefit Cost
Service cost	4,409	4,789	4,265
Interest cost	5,851	6,450	6,651
Amortization of prior service credit	(1,372)	(674)	(728)
Amortization of actuarial loss	1,346	1,149	887
Net periodic pension benefit cost	10,234	11,714	11,075
Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income
Net actuarial (gain) loss	13,562	(2,853)	11,730
Prior service credit	(38,414)	0	0
Reclassification of actuarial loss to net periodic postretirement benefit cost	(1,346)	(1,149)	(887)
Reclassification of prior service credit to net periodic postretirement benefit cost	1,372	674	728
Amount recognized in other comprehensive income	(24,826)	(3,328)	11,571
Amount recognized in net periodic postretirement benefit cost and other comprehensive income	$ (14,592)	$ 8,386	$ 22,646
Assumed Healthcare Cost Trend Rates at December 31
Healthcare cost trend rate assumed for next year	8.00%	7.50%	8.00%
Rate to which the cost trend rate gradually declines	5.00%	5.00%	5.00%
Year that the rate reaches the rate it is assumed to maintain	2019	2017	2017
Weighted-average assumptions used to determine net periodic benefit cost for years ended December 31
Discount rate	4.60%	4.95%	5.45%
Weighted-average assumptions used to determine benefit obligation at December 31
Discount rate	3.30%	4.60%	4.95%