Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets
Interest rate swap agreements		$ 3
Total assets	$ 271,435	276,618
Liabilities
Accrued expenses and other current liabilities	697	996
Total liabilities	30,507	23,381
Money market funds
Assets
Cash equivalents:	271,435	276,615
Natural gas fixed price forward contracts
Liabilities
Derivatives	6,503	6,968
Embedded EPP derivatives
Liabilities
Derivatives	5,892	6,176
Interest rate swap agreements
Liabilities
Derivatives	17,415	9,241
Level 1
Assets
Interest rate swap agreements		0
Total assets	271,435	276,615
Liabilities
Accrued expenses and other current liabilities	697	996
Total liabilities	697	996
Level 1 | Money market funds
Assets
Cash equivalents:	271,435	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	0
Total liabilities	17,415	9,241
Level 2 | Money market funds
Assets
Cash equivalents:	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	17,415	9,241
Level 3
Assets
Interest rate swap agreements		0
Total assets	0	0
Liabilities
Accrued expenses and other current liabilities	0	0
Total liabilities	12,395	13,144
Level 3 | Money market funds
Assets
Cash equivalents:	0	0
Level 3 | Natural gas fixed price forward contracts
Liabilities
Derivatives	6,503	6,968
Level 3 | Embedded EPP derivatives
Liabilities
Derivatives	5,892	6,176
Level 3 | Interest rate swap agreements
Liabilities
Derivatives	$ 0	$ 0