Summary of Significant Accounting Policies - Summary of Revisions in Risk-Free Discount Rate Applied to Environmental Remediation Liabilities and Recovery Assets (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Environmental Exit Cost [Line Items]
Charge to Operating expenses	$ 3	$ 17	$ 2
Risk-free discount rate applied to environmental remediation liabilities and recovery assets	1.75%	2.00%	3.50%