Fair Value of Financial Instruments - Financial Instruments Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Inputs, Level 1 | Gas Derivatives
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Gas Derivatives	$ 0	$ 0
Fair Value, Inputs, Level 1 | Interest Rate Swaps
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Gas Derivatives	0	0
Fair Value, Inputs, Level 2 | Gas Derivatives
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Gas Derivatives	369,762	405,781
Fair Value, Inputs, Level 2 | Interest Rate Swaps
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Gas Derivatives	(11,858)	(1,219)
Fair Value, Inputs, Level 3 | Gas Derivatives
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Gas Derivatives	0	0
Fair Value, Inputs, Level 3 | Interest Rate Swaps
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Gas Derivatives	$ 0	$ 0