Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets
Interest rate swap agreements		$ 3
Total assets	$ 235,902	276,618
Liabilities
Accrued expenses and other current liabilities		996
Total liabilities	24,104	23,381
Money market funds
Assets
Money market funds	235,902	276,615
Natural gas fixed price forward contracts
Liabilities
Derivatives:	2,574	6,968
Embedded EPP derivatives
Liabilities
Derivatives:	5,541	6,176
Interest rate swap agreements
Liabilities
Derivatives:	15,989	9,241
Level 1
Assets
Interest rate swap agreements		0
Total assets	235,902	276,615
Liabilities
Accrued expenses and other current liabilities		996
Total liabilities	0	996
Level 1 | Money market funds
Assets
Money market funds	235,902	276,615
Level 1 | Natural gas fixed price forward contracts
Liabilities
Derivatives:	0	0
Level 1 | Embedded EPP derivatives
Liabilities
Derivatives:	0	0
Level 1 | Interest rate swap agreements
Liabilities
Derivatives:	0	0
Level 2
Assets
Interest rate swap agreements		3
Total assets	0	3
Liabilities
Accrued expenses and other current liabilities		0
Total liabilities	15,989	9,241
Level 2 | Money market funds
Assets
Money market funds	0	0
Level 2 | Natural gas fixed price forward contracts
Liabilities
Derivatives:	0	0
Level 2 | Embedded EPP derivatives
Liabilities
Derivatives:	0	0
Level 2 | Interest rate swap agreements
Liabilities
Derivatives:	15,989	9,241
Level 3
Assets
Interest rate swap agreements		0
Total assets	0	0
Liabilities
Accrued expenses and other current liabilities		0
Total liabilities	8,115	13,144
Level 3 | Money market funds
Assets
Money market funds	0	0
Level 3 | Natural gas fixed price forward contracts
Liabilities
Derivatives:	2,574	6,968
Level 3 | Embedded EPP derivatives
Liabilities
Derivatives:	5,541	6,176
Level 3 | Interest rate swap agreements
Liabilities
Derivatives:	$ 0	$ 0