Derivative Instruments and Hedging Activities - Outstanding Commodity Derivatives (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2019 $ / Mcf Bcf	Dec. 31, 2018 $ / Mcf Bcf
Natural gas (IFERC TRANSCO Z6 non-NY)
Derivative [Line Items]
Volume (Mmbtu) | Bcf		10,950,000
Natural gas (IFERC TRANSCO Z6 non-NY)
Derivative [Line Items]
Volume (Mmbtu) | Bcf		11,700,000
Swaps Weighted Average (in usd per Mmbtu) | $ / Mcf		7.38
Natural gas (IFERC TRANSCO Leidy Line Receipts)
Derivative [Line Items]
Volume (Mmbtu) | Bcf		54,750,000
Natural gas (NYMEX)
Derivative [Line Items]
Volume (Mmbtu) | Bcf		4,500,000
Swaps Weighted Average (in usd per Mmbtu) | $ / Mcf		4.31
Natural gas (NYMEX)
Derivative [Line Items]
Volume (Mmbtu) | Bcf		10,700,000
Swaps Weighted Average (in usd per Mmbtu) | $ / Mcf		2.75
Natural gas (NYMEX)
Derivative [Line Items]
Volume (Mmbtu) | Bcf		109,500,000
Swaps Weighted Average (in usd per Mmbtu) | $ / Mcf		3.13
Weighted Average | Natural gas (IFERC TRANSCO Z6 non-NY)
Derivative [Line Items]
Swaps Weighted Average (in usd per Mmbtu) | $ / Mcf		0.41
Weighted Average | Natural gas (IFERC TRANSCO Leidy Line Receipts)
Derivative [Line Items]
Swaps Weighted Average (in usd per Mmbtu) | $ / Mcf		(0.53)
Subsequent Event | Natural gas (NYMEX)
Derivative [Line Items]
Volume (Mmbtu) | Bcf	42,800,000
Swaps Weighted Average (in usd per Mmbtu) | $ / Mcf	2.86