Fair Value Measurements (Carrying Amount and Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Millions	1 Months Ended				12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jul. 31, 2016	Jan. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, net					$ 76	$ (375)
Repayments of long-term debt					1,139	1,175	$ 500
Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents					916	1,423
Derivative instruments, net					76	(375)
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents					916	1,423
Derivative instruments, net					76	(375)
Unsecured 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
Unsecured 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
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	2020-12
Accelerated maturity date, year and month if not amended, redeemed or refinanced					2019-10
Repayments of long-term debt	$ 327	$ 48	$ 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
Amount to be amended, redeemed or refinanced to avoid acceleration					$ 765	$ 765
Repayments of long-term debt	$ 758				758
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] | 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
Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repayments of long-term debt			$ 700
Senior Notes [Member] | Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes					3,242	3,166
Senior Notes [Member] | Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes					3,358	$ 3,182
Purchased Call Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Premium paid					$ 1