Derivative Instruments - Commodity derivatives (Details) $ in Thousands	Dec. 31, 2018 USD ($) MMBTU / d $ / MMBTU	Dec. 31, 2017 USD ($)
Derivatives not designated as hedges for accounting purposes | NYMEX | Three Months Ending June302019 [Member]
Derivative Instruments
Notional amount (MMBtu/Bbls per day) | MMBTU / d	1,575,000
Weighted average index price ceiling price	3.30
Weighted average index price floor price	2.50
Derivatives not designated as hedges for accounting purposes | NYMEX | Three Months Ending September302019 [Member]
Derivative Instruments
Notional amount (MMBtu/Bbls per day) | MMBTU / d	1,575,000
Weighted average index price ceiling price	3.30
Weighted average index price floor price	2.50
Derivatives not designated as hedges for accounting purposes | NYMEX | Three Months Ending December312019 [Member]
Derivative Instruments
Notional amount (MMBtu/Bbls per day) | MMBTU / d	1,575,000
Weighted average index price ceiling price	3.52
Weighted average index price floor price	2.50
Commodity derivative
Derivative Instruments
Gross amounts on balance sheet | $	$ 658,830	$ 1,367,554
Commodity derivative | Derivatives not designated as hedges for accounting purposes
Derivative Instruments
Notional amount (MMBtu/Bbls per day) | MMBTU / d	225,000
Gross amounts on balance sheet | $	$ 13,000
Commodity derivative | Derivatives not designated as hedges for accounting purposes | Basis Differential Positions | Minimum
Derivative Instruments
Weighted average index price	0.215
Commodity derivative | Derivatives not designated as hedges for accounting purposes | Basis Differential Positions | Maximum
Derivative Instruments
Weighted average index price	0.40
Swaps | Natural gas | NYMEX | Three Months Ending March312019 [Member]
Derivative Instruments
Notional amount (MMBtu/Bbls per day) | MMBTU / d	2,330,000
Weighted average index price	3.62
Swaps | Natural gas | NYMEX | Three Months Ending June302019 [Member]
Derivative Instruments
Notional amount (MMBtu/Bbls per day) | MMBTU / d	755,000
Weighted average index price	3.26
Swaps | Natural gas | NYMEX | Three Months Ending September302019 [Member]
Derivative Instruments
Notional amount (MMBtu/Bbls per day) | MMBTU / d	755,000
Weighted average index price	3.32
Swaps | Natural gas | NYMEX | Three Months Ending December312019 [Member]
Derivative Instruments
Notional amount (MMBtu/Bbls per day) | MMBTU / d	755,000
Weighted average index price	3.45
Swaps | Natural gas | NYMEX | Year ending December 31, 2020
Derivative Instruments
Notional amount (MMBtu/Bbls per day) | MMBTU / d	1,417,500
Weighted average index price	3.00
Swaps | Natural gas | NYMEX | Year Ending December 31, 2021
Derivative Instruments
Notional amount (MMBtu/Bbls per day) | MMBTU / d	710,000
Weighted average index price	3.00
Swaps | Natural gas | NYMEX | Year ending December 31, 2022
Derivative Instruments
Notional amount (MMBtu/Bbls per day) | MMBTU / d	850,000
Weighted average index price	3.00
Swaps | Natural gas | NYMEX | Year ending December 31, 2023
Derivative Instruments
Notional amount (MMBtu/Bbls per day) | MMBTU / d	90,000
Weighted average index price	2.91