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Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 segment	Dec. 31, 2011	Dec. 31, 2010
Accounting Policies [Abstract]
Depreciation expense	$ 104	$ 89	$ 80
Number of reportable segments	1

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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