Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for doubtful accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Balance at beginning of period		$ 26	$ 21	$ 60
Valuation Allowances and Reserves, Charged to Cost and Expense		24	11	10
Valuation Allowances and Reserves, Deductions		(7)	(7)	(5)
Valuation Allowances and Reserves, Other Activity		(1)	1	(44)	[1]
Valuation Allowances and Reserves, Balance at end of period		42	26	21
Tax valuation allowance
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Balance at beginning of period		910	747	642
Valuation Allowances and Reserves, Charged to Cost and Expense	[2]	578	192	187
Valuation Allowances and Reserves, Deductions		0	0	(15)
Valuation Allowances and Reserves, Other Activity		(30)	(29)	(67)
Valuation Allowances and Reserves, Balance at end of period		$ 1,458	$ 910	$ 747

[1] Other Activity primarily represents the reclassification of balances related to billing adjustments to accounts receivable.

[2] Additions Charged to Costs and Expenses are primarily related to taxable losses for which the tax benefit has been reserved.