Changes in Accumulated OCI (Loss) by Component (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 10,958,229	$ 10,958,229	$ 18,414,613	$ 9,119,247
(Gains) losses reclassified from accumulated OCI, net of tax		703	(3,851)	(4,500)
Distribution to Equitrans Midstream Corporation			903
Ending Balance		9,803,588	10,958,229	18,414,613
Cash flow hedge, net of tax | Natural gas cash flow hedges, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	0	0	4,625	9,607
(Gains) losses reclassified from accumulated OCI, net of tax		0	(4,625)	(4,982)
Distribution to Equitrans Midstream Corporation			0
Change in accounting principle	0
Ending Balance		0	0	4,625
Cash flow hedge, net of tax | Interest rate cash flow hedges, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(387)	(387)	(555)	(699)
(Gains) losses reclassified from accumulated OCI, net of tax		387	168	144
Distribution to Equitrans Midstream Corporation			0
Change in accounting principle	0
Ending Balance		0	(387)	(555)
Other postretirement benefits liability adjustment, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(5,019)	(5,019)	(6,528)	(6,866)
(Gains) losses reclassified from accumulated OCI, net of tax		316	606	338
Distribution to Equitrans Midstream Corporation			903
Change in accounting principle	(496)
Ending Balance		(5,199)	(5,019)	(6,528)
Accumulated OCI (loss), net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(5,406)	(5,406)	(2,458)	2,042
Change in accounting principle	$ (496)
Ending Balance		$ (5,199)	$ (5,406)	$ (2,458)