Income Taxes - Summary of Source and Tax Effects of Temporary Differences between Financial Reporting and Tax Bases of Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred income taxes:
Total deferred income tax assets	$ (971,184)	$ (875,303)
Total deferred income tax liabilities	2,740,084	2,635,307
Total net deferred income tax liabilities	1,768,900	1,760,004
Total deferred income tax liabilities (assets):
Drilling and development costs expensed for income tax reporting	2,074,091	1,473,355
Tax depreciation in excess of book depreciation	644,590	1,161,952
Incentive compensation and deferred compensation plans	(43,822)	(77,743)
Net operating loss carryforwards	(564,180)	(282,943)
Investment in partnerships	(132,667)	(386,676)
Alternative minimum tax credit carryforward	(435,190)	(224,428)
Federal tax credits	(50,341)	(2,508)
Unrealized hedge (losses) gains	21,403	(101,430)
Other	(7,376)	(997)
Total excluding valuation allowances	1,506,508	1,558,582
Valuation allowances	$ 262,392	$ 201,422