Property, Plant and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 8,610,000	$ 8,160,000
Property, plant and equipment, net	5,095,880	5,078,650
Group Of Wells Assets With Carrying Value Closest To Its Undiscounted Cash Flows [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net	74,000	$ 80,800
Other Groups of Wells Assets [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net	$ 183,900