Summary of Significant Accounting Policies Summary of Significant Accounting Policies - Allowance for Doubtful Accounts (Details) - Allowance for Doubtful Accounts, Current [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Valuation Allowance [Line Items]
Valuation Allowances and Reserves, Beginning Balance	$ 48	$ 196	$ 162	$ 202
Valuation Allowances and Reserves, Charged to Cost and Expense	11	50	29
Valuation Allowances and Reserves, Deductions	3	(16)	(69)
Valuation Allowances and Reserves, Adjustments	(162)	0	0
Valuation Allowances and Reserves, Ending Balance	$ 48	$ 196	$ 162	$ 202