Derivative Instruments and Hedging Activities - Outstanding Commodity Derivatives (Details) Bcf in Millions	3 Months Ended
	Mar. 31, 2018 $ / Mcf Bcf
Natural gas (Leidy)
Derivative [Line Items]
Volume (in Bcf for gas and mmbbls for oil) | Bcf	26.7
Swaps Weighted Average (in usd per Mcf or usd per bbl) | $ / Mcf	(0.69)
Natural gas (Transco)
Derivative [Line Items]
Volume (in Bcf for gas and mmbbls for oil) | Bcf	18.7
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	69.5
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	10.4
Swaps Weighted Average (in usd per Mcf or usd per bbl) | $ / Mcf	3.10
Natural gas purchase
Derivative [Line Items]
Volume (in Bcf for gas and mmbbls for oil) | Bcf	0.0
Swaps Weighted Average (in usd per Mcf or usd per bbl) | $ / Mcf	3.67