Summary of Accounting Policies - Notes and Accounts Receivable and Allowance for Doubtful Accounts (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Accounting Policies [Abstract]
Short-term and long-term accounts receivable outstanding	$ 1,088	$ 1,065
Allowance for doubtful accounts on short-term and long-term accounts receivable	16	14
Short and Long-term notes receivable outstanding		5
Allowance for doubtful accounts on short-term and long-term notes receivable	$ 0	$ 0