Fair Value of Financial Instruments (The Waha JVs) (Narrative) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) pipeline	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) pipeline	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Equity method investments, equity in earnings (losses)	$ 6,551	$ 5,824	$ 12,811	$ 11,409
Unrealized gains (losses) on equity investee activity, net of tax	(8,732)	2,937	(14,194)	10,653
Equity Investee [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Equity method investments, net investment	$ 185,000		$ 185,000		$ 197,000
Waha JVs [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Number of pipelines in operation | pipeline	2		2
Equity method investments, equity contributions	$ 600	9,700	$ 1,300	24,500
Equity method investments, equity in earnings (losses)	6,600	5,800	12,900	11,400
Equity method investments, cumulative undistributed earnings	36,800		36,800
Equity method investments, distributions of earnings received, operating cash flows	2,100	2,000	6,000	7,700
Unrealized gains (losses) on equity investee activity, before tax	(11,600)	3,900	(18,800)	14,000
Unrealized gains (losses) on equity investee activity, net of tax	(8,700)	$ 2,900	(14,200)	$ 10,700
Waha JVs [Member] | Equity Investee [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Equity method investments, net investment	$ 164,000		$ 164,000		$ 168,000