Fair Value of Financial Instruments (Equity Investments) (Narrative) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Equity investments, adjusted cost basis, amount	$ 18,000,000	$ 18,000,000
Equity investments, upward price adjustment, amount	0
Equity investments, downward price adjustment, amount	0
Equity investments, impairment, amount	0
Equity Investee [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Equity investments, carrying value	197,000,000	141,000,000
CCI [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Equity investments, adjusted cost basis, amount	$ 15,000,000
Waha JVs [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Equity method investment, ownership percentage	33.00%
Waha JVs [Member] | Equity Investee [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Equity investments, carrying value	$ 168,000,000	$ 121,000,000