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, 2018	Dec. 31, 2017
Deferred income taxes:
Total deferred income tax assets	$ (901,377)	$ (1,112,514)
Total deferred income tax liabilities	2,724,758	3,002,476
Total net deferred income tax liabilities	1,823,381	1,889,962
Total deferred income tax liabilities (assets):
Drilling and development costs expensed for income tax reporting	1,469,320	2,074,091
Tax depreciation in excess of book depreciation	904,030	644,590
Investment in Equitrans Midstream	(10,359)	0
Incentive compensation and deferred compensation plans	(24,682)	(43,822)
Net operating loss carryforwards	(429,983)	(564,180)
Alternative minimum tax credit carryforward	(308,727)	(435,190)
Federal tax credits	(37,710)	(50,341)
Unrealized (losses) gains	(28,096)	21,403
Interest disallowance limitation	(35,358)	0
Other	(26,462)	(18,981)
Total excluding valuation allowances	1,471,973	1,627,570
Valuation allowances	$ 351,408	$ 262,392