Equity Investments - Summarized Combined Financial Data For Equity Investments - Balance Sheet Table (Details) - Western Gas Partners, LP [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Current assets	$ 137,957	$ 118,472
Property, plant and equipment, net	2,512,214	2,626,466
Other assets	36,373	39,802
Total assets	2,686,544	2,784,740
Current liabilities	80,490	63,468
Non-current liabilities	7,447	6,662
Equity	2,598,607	2,714,610
Total liabilities and equity	$ 2,686,544	$ 2,784,740