Equity Investments - Summarized Combined Financial Data For Equity Investments - Income Statement Table (Details) - Western Gas Partners, LP [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Revenues	$ 703,424	$ 687,554	$ 667,554
Operating income	435,735	428,454	359,899
Net income	$ 434,749	$ 427,511	$ 359,443