Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes [Line Items]
Alternative minimum tax ("AMT") credit carryforwards	$ 232		$ 232	$ 214
Income tax expense (benefit)			(3,121)	(4)	$ (32)
Income tax benefit associated with valuation allowance release	88	$ (3,218)	(3,130)
Pre-tax profit benefited from lower oil prices and improved efficiency			4,219	1,128	539
Valuation allowance	48		$ 48	4,751
Federal statutory income tax rate			35.00%
Unrecognized tax benefits	24		$ 24	$ 9	$ 14
Unrecognized tax benefits that would impact effective tax rate if recognized	21		21
Derivative
Income Taxes [Line Items]
Valuation allowance	180		$ 180
Minimum
Income Taxes [Line Items]
State tax benefits of the NOL carry forwards expiration period, years			5 years
Maximum
Income Taxes [Line Items]
State tax benefits of the NOL carry forwards expiration period, years			20 years
Pension Benefits
Income Taxes [Line Items]
Valuation allowance	285		$ 285
2025
Income Taxes [Line Items]
Net operating loss expiration amount	2,100		2,100
2026
Income Taxes [Line Items]
Net operating loss expiration amount	2,000		2,000
2027
Income Taxes [Line Items]
Net operating loss expiration amount	1,400		1,400
Thereafter
Income Taxes [Line Items]
Net operating loss expiration amount	2,500		2,500
Valuation Allowance Reversal | Current year income
Income Taxes [Line Items]
Valuation allowance release			1,500
Valuation Allowance Reversal | Gross Amount
Income Taxes [Line Items]
Valuation allowance release			4,700
Federal
Income Taxes [Line Items]
NOL carry forwards	8,000		8,000
State
Income Taxes [Line Items]
NOL carry forwards	103		103
Valuation allowance	$ 48		$ 48