Property, Plant and Equipment - Additional Information (Detail) $ in Thousands	Oct. 01, 2016 USD ($) a	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 8,160,000	$ 7,740,000
Property, plant and equipment, net	$ 4,913,355	4,891,153
South Piceance Basin [Member]
Property, Plant and Equipment [Line Items]
Percentage of leasehold interest in unproved oil and gas properties	49.00%
Area of leasehold interest in unproved oil and gas properties | a	54,000
Cost of leasehold interest in unproved oil and gas properties	$ 165,000
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	$ 78,300	$ 87,200