Fair Value of Financial Instruments - Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Inputs, Level 1 | Gas Derivatives
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Derivative assets	$ 0	$ 0
Fair Value, Inputs, Level 1 | Interest Rate Swaps
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Derivative assets	0	0
Fair Value, Inputs, Level 2 | Gas Derivatives
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Derivative assets	(94,939)	405,781
Fair Value, Inputs, Level 2 | Interest Rate Swaps
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Derivative assets	(15,609)	(1,219)
Fair Value, Inputs, Level 3 | Gas Derivatives
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Derivative assets	0	0
Fair Value, Inputs, Level 3 | Interest Rate Swaps
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Derivative assets	$ 0	$ 0