Fair Value of Financial Instruments (The Waha JVs) (Narrative) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) pipeline	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) pipeline	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Letters of credit issued	$ 106,100,000		$ 106,100,000		$ 157,100,000
Equity method investments, equity in earnings (losses)	7,671,000	$ 7,399,000	19,080,000	$ 15,105,000
Unrealized gains (losses) on equity method investee activity, net of tax	3,137,000	808,000	13,790,000	(1,287,000)
Equity Investee [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Equity method investments, net investment	197,000,000		197,000,000		141,000,000
Credit Facility [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Letters of credit issued	$ 89,000,000		$ 89,000,000		157,100,000
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	$ 0	0	$ 24,500,000	73,300,000
Equity method investments, equity in earnings (losses)	7,700,000	7,400,000	19,100,000	15,100,000
Equity method investments, cumulative undistributed earnings	25,100,000		25,100,000
Equity method investments, distributions of earnings received	3,200,000		10,900,000
Revenue, related parties		3,600,000		255,200,000
Receivables, net of BIEC, related parties	100,000		100,000		2,800,000
Unrealized gains (losses) on equity method investee activity	4,100,000	1,300,000	18,100,000	(2,100,000)
Unrealized gains (losses) on equity method investee activity, net of tax	3,100,000	$ 800,000	13,800,000	$ (1,300,000)
Waha JVs [Member] | Equity Investee [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Equity method investments, net investment	168,000,000		168,000,000		121,000,000
Waha JVs [Member] | Credit Facility [Member]
Fair Value, Financial Instruments Measured on a Non-Recurring Basis [Line Items]
Letters of credit issued	$ 0		$ 0		$ 19,000,000