Long-Term Debt (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Senior Notes [Member]
Debt Instrument [Line Items]
Extinguishment of debt		$ 933	$ 793
Senior Notes [Member] | Change Of Control And Loss Of Investment Grade Rating [Member]
Debt Instrument [Line Items]
Required purchase price as percentage of aggregate principal amount	101.00%
Reported Value Measurement [Member]
Debt Instrument [Line Items]
Fair value of long term debt	$ 13,900	13,900
Level 2 [Member] | Estimate of Fair Value Measurement [Member]
Debt Instrument [Line Items]
Fair value of long term debt	$ 15,300	$ 15,100