Fair Value Disclosures (Estimated Fair Values and Carrying Amounts of Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, carrying value	$ 196	$ 182	$ 288	$ 245	$ 277	$ 323	$ 206	$ 220	$ 314
Cash and cash equivalents, fair value	196				277
Restricted cash, carrying value	142				136
Restricted Cash Fair Value Disclosure	142				136
Debt and other obligations, carrying amount	18,121				16,682
Obligations, Fair Value Disclosure	$ 19,170				$ 16,562