Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Balance at beginning of period		$ 33	$ 18	$ 13
Valuation Allowances and Reserves, Charged to Cost and Expense		23	22	6
Valuation Allowances and Reserves, Deductions		(24)	(6)	(3)
Valuation Allowances and Reserves, Other Activity		2	(1)	2
Valuation Allowances and Reserves, Balance at end of period		34	33	18
Tax valuation allowance
Valuation and Qualifying Accounts Disclosure [Line Items]
Valuation Allowances and Reserves, Balance at beginning of period		1,399	706	507
Valuation Allowances and Reserves, Charged to Cost and Expense	[1]	0	706	187
Valuation Allowances and Reserves, Deductions		(406)	0	0
Valuation Allowances and Reserves, Other Activity		15	(13)	12
Valuation Allowances and Reserves, Balance at end of period		$ 1,008	$ 1,399	$ 706

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