Employee Benefit Plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Current year actuarial loss (gain)	$ 114,061	$ 311,038	$ 60,777
Recognition of actuarial loss	(70,161)	(53,039)	(35,264)
Current year prior service (credit) cost	(4,217)		1,148
Recognition of prior service credit	30,777	6,970	6,979
Total recognized in other comprehensive income	70,460	264,969	33,640
Total recognized in net periodic benefit cost and other comprehensive income	97,228	297,220	55,524
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Current year actuarial loss (gain)	(371)	(3,911)	340
Recognition of actuarial loss	(1,254)	(1,708)	(1,759)
Current year prior service (credit) cost		362
Recognition of prior service credit	930	930	1,059
Total recognized in other comprehensive income	(695)	(4,327)	(360)
Total recognized in net periodic benefit cost and other comprehensive income	$ (104)	$ (3,075)	$ 945