Analysis of Deferred Tax Assets and Deferred Tax Liabilities Showing Tax Effects of Significant Temporary Differences (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred tax assets
Property and leasehold costs	$ 658,588	$ 627,093
Liabilities for dismantlements	78,100	114,175
Postretirement and other employee benefits	197,912	164,044
Alternative minimum tax	37,253	0
Foreign tax credit carryforwards	18,594	21,368
Other deferred tax assets	32,500	38,341
Total gross deferred tax assets	1,022,947	965,021
Less valuation allowance	(523,966)	(445,842)
Net deferred tax assets	498,981	519,179
Deferred tax liabilities
Property, plant and equipment	(808,311)	(851,330)
Accumulated depreciation, depletion and amortization	(1,077,867)	(754,295)
Other deferred tax liabilities	(70,710)	(78,751)
Total gross deferred tax liabilities	(1,956,888)	(1,684,376)
Net deferred tax liabilities	$ (1,457,907)	$ (1,165,197)