Fair Value Disclosures (Estimated Fair Values and Carrying Amounts of Assets and Liabilities) (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, at carrying value		$ 220	$ 314
Cash and cash equivalents, fair value		220	314
Cash and Cash Equivalents, at Carrying Value, Including Discontinued Operations	[1]	345	440	$ 325	$ 697
Restricted cash, current and non-current, carrying value		125	126
Restricted cash, current and non-current, fair value		125	126
Long-term debt and other obligations, carrying amount		15,746	16,159
Long-term debt and other obligations, fair value		$ 15,870	$ 16,644

[1] See "Recently Adopted Accounting Pronouncements" in note 2 to the condensed consolidated financial statements for a discussion of recently adopted restricted cash guidance, which impacted certain presentations on the condensed consolidated statement of cash flows.