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, 2011	Dec. 31, 2010
Deferred tax assets
Property and leasehold costs	$ 627,093	$ 391,168
Liabilities for dismantlements	114,175	97,149
Postretirement and other employee benefits	164,044	137,709
Foreign tax credit carryforwards	21,368	47,725
Other deferred tax assets	38,341	47,511
Total gross deferred tax assets	965,021	721,262
Less valuation allowance	(445,842)	(305,349)
Net deferred tax assets	519,179	415,913
Deferred tax liabilities
Property, plant and equipment	(851,330)	(729,699)
Accumulated depreciation, depletion and amortization	(754,295)	(702,512)
Deferred major repair costs	(11,257)	(35,848)
Other deferred tax liabilities	(67,494)	(96,838)
Total gross deferred tax liabilities	(1,684,376)	(1,564,897)
Net deferred tax liabilities	$ (1,165,197)	$ (1,148,984)