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, 2020	Dec. 31, 2019
Deferred income taxes:
Total deferred income tax assets	$ (610,821)	$ (643,227)
Total deferred income tax liabilities	1,982,788	2,129,041
Total net deferred income tax liabilities	1,371,967	1,485,814
Total deferred income tax liabilities (assets):
Drilling and development costs expensed for income tax reporting	918,120	1,100,061
Tax depreciation in excess of book depreciation	1,027,179	974,520
Investment in Equitrans Midstream	(94,689)	(109,883)
Incentive compensation and deferred compensation plans	(22,419)	(16,923)
NOL carryforwards	(789,544)	(635,446)
Alternative minimum tax credit carryforward	(81,237)	(190,992)
Federal tax credits	(79,846)	(59,854)
State capital loss carryforward	(28,317)	0
Unrealized (losses) gains	(43,475)	54,460
Interest disallowance limitation	(160)	(46,776)
Convertible debt	37,489	0
Other	(1,126)	(6,797)
Total excluding valuation allowances	841,975	1,062,370
Valuation allowances	529,992	423,444
Total net deferred income tax liabilities	$ 1,371,967	$ 1,485,814