Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets
Interest rate swap agreements		$ 3
Total assets	$ 236,037	276,618
Liabilities
Accrued other liabilities	1,451	996
Total liabilities	29,997	23,381
Money market funds
Assets
Cash equivalents:	236,037	276,615
Natural gas fixed price forward contracts
Liabilities
Derivatives:	5,185	6,968
Embedded derivative on 6% promissory notes
Liabilities
Derivatives:	5,480	6,176
Interest rate swap agreements
Liabilities
Derivatives:	17,881	9,241
Level 1
Assets
Interest rate swap agreements		0
Total assets	236,037	276,615
Liabilities
Accrued other liabilities	1,451	996
Total liabilities	1,451	996
Level 1 | Money market funds
Assets
Cash equivalents:	236,037	276,615
Level 1 | Natural gas fixed price forward contracts
Liabilities
Derivatives:	0	0
Level 1 | Embedded derivative on 6% promissory notes
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 other liabilities	0	0
Total liabilities	17,881	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 derivative on 6% promissory notes
Liabilities
Derivatives:	0	0
Level 2 | Interest rate swap agreements
Liabilities
Derivatives:	17,881	9,241
Level 3
Assets
Interest rate swap agreements		0
Total assets	0	0
Liabilities
Accrued other liabilities	0	0
Total liabilities	10,665	13,144
Level 3 | Money market funds
Assets
Cash equivalents:	0	0
Level 3 | Natural gas fixed price forward contracts
Liabilities
Derivatives:	5,185	6,968
Level 3 | Embedded derivative on 6% promissory notes
Liabilities
Derivatives:	5,480	6,176
Level 3 | Interest rate swap agreements
Liabilities
Derivatives:	$ 0	$ 0