Fair Value Measurements (Carrying Amount And Estimated Fair Values Of Financial Instruments) (Details) - USD ($) $ in Millions	1 Months Ended			3 Months Ended	9 Months Ended		12 Months Ended	21 Months Ended
	Sep. 30, 2017	Jul. 31, 2016	Jan. 31, 2015	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, net	$ (25)			$ (25)	$ (25)		$ (375)	$ (25)
Repayments of long-term debt					1,139	$ 1,175
Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	989			989	989		1,423	989
Senior notes	3,282			3,282	3,282		3,166	3,282
Derivative instruments, net	(25)			(25)	(25)		(375)	(25)
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	989			989	989		1,423	989
Senior notes	3,317			3,317	3,317		3,182	3,317
Derivative instruments, net	(25)			(25)	(25)		$ (375)	(25)
Long-term Debt [Member] | 2015 Term Loan due December 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity date, year and month							2020-12
Repayments of long-term debt	327	$ 375			$ 327		$ 423
Long-term Debt [Member] | 2016 Term Loan due December 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity date, year and month					2020-12		2020-12
Accelerated maturity date, year and month if not amended, redeemed or refinanced					2019-10
Long-term Debt [Member] | 4.05% Senior Notes due January 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity date, year and month			2020-01		2020-01		2020-01
Repayments of long-term debt	758				$ 758
Long-term Debt [Member] | 2020 Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount to be amended, redeemed or refinanced to avoid acceleration					765			765
Long-term Debt [Member] | Carrying Amount [Member] | 2015 Term Loan due December 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans							$ 327
Long-term Debt [Member] | Carrying Amount [Member] | 2016 Term Loan due December 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	1,191			1,191	1,191		1,191	1,191
Long-term Debt [Member] | Fair Value [Member] | 2015 Term Loan due December 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans							327
Long-term Debt [Member] | Fair Value [Member] | 2016 Term Loan due December 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	$ 1,191			1,191	1,191		1,191	$ 1,191
Call Options - 2017 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Premium paid				$ 3	3
Call Options - 2017 [Member] | Not Designated as Hedging Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Premium paid					$ 2		$ 2