Derivative Instruments and Hedging Activities - Outstanding Commodity Derivatives (Details) Bcf in Millions	9 Months Ended
	Sep. 30, 2018 $ / Mcf Bcf
Natural gas (Leidy)
Derivative [Line Items]
Volume (in Bcf for gas and mmbbls for oil) | Bcf	8.9
Swaps Weighted Average (in usd per Mcf or usd per bbl) | $ / Mcf	(0.69)
Natural gas (Leidy)
Derivative [Line Items]
Volume (in Bcf for gas and mmbbls for oil) | Bcf	53.2
Swaps Weighted Average (in usd per Mcf or usd per bbl) | $ / Mcf	(0.55)
Natural gas (Transco)
Derivative [Line Items]
Volume (in Bcf for gas and mmbbls for oil) | Bcf	13.3
Swaps Weighted Average (in usd per Mcf or usd per bbl) | $ / Mcf	0.42
Natural gas (NYMEX)
Derivative [Line Items]
Volume (in Bcf for gas and mmbbls for oil) | Bcf	23.2
Swaps Weighted Average (in usd per Mcf or usd per bbl) | $ / Mcf	2.93
Natural gas (NYMEX)
Derivative [Line Items]
Volume (in Bcf for gas and mmbbls for oil) | Bcf	1.5
Swaps Weighted Average (in usd per Mcf or usd per bbl) | $ / Mcf	3.10