Reclamation and Remediation - Reclamation and Remediation Expense (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Schedule Of Accrual For Asset Retirement Obligations [Line Items]
Accretion expense	$ 21	$ 18	$ 41	$ 36
Operating Expense [Member]
Schedule Of Accrual For Asset Retirement Obligations [Line Items]
Accretion expense	18	15	36	30
Non-Operating [Member]
Schedule Of Accrual For Asset Retirement Obligations [Line Items]
Accretion expense	$ 3	$ 3	$ 5	$ 6