Summary of Significant Accounting Policies - Credit Risk and Concentration (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Concentration Risk
Bad debt expense	$ 9	$ 8	$ 9
Major customer percentage	10.00%	10.00%	10.00%
Customer concentration risk
Concentration Risk
Number of major customer	0	1	0
Customer concentration risk | Sales revenue
Concentration Risk
Entity-Wide Revenue, Major Customer, Percentage		12.00%
Geographic concentration risk | Sales revenue | U.S.
Concentration Risk
Concentration risk percentage	51.00%	56.00%	62.00%