Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets
Short-term investments		$ 104,350
Interest rate swap agreements	$ 3	82
Total assets	276,618	248,275
Liabilities
Accrued other liabilities	996	1,331
Total liabilities	23,381	18,705
Money market funds
Assets
Cash equivalents:	276,615	143,843
Natural gas fixed price forward contracts
Liabilities
Derivatives	6,968	9,729
Embedded EPP derivatives
Liabilities
Derivatives	6,176	4,015
Interest rate swap agreements
Liabilities
Derivatives	9,241	3,630
Level 1
Assets
Short-term investments		104,350
Interest rate swap agreements	0	0
Total assets	276,615	248,193
Liabilities
Accrued other liabilities	996	1,331
Total liabilities	996	1,331
Level 1 | Money market funds
Assets
Cash equivalents:	276,615	143,843
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
Short-term investments		0
Interest rate swap agreements	3	82
Total assets	3	82
Liabilities
Accrued other liabilities	0	0
Total liabilities	9,241	3,630
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	9,241	3,630
Level 3
Assets
Short-term investments		0
Interest rate swap agreements	0	0
Total assets	0	0
Liabilities
Accrued other liabilities	0	0
Total liabilities	13,144	13,744
Level 3 | Money market funds
Assets
Cash equivalents:	0	0
Level 3 | Natural gas fixed price forward contracts
Liabilities
Derivatives	6,968	9,729
Level 3 | Embedded EPP derivatives
Liabilities
Derivatives	6,176	4,015
Level 3 | Interest rate swap agreements
Liabilities
Derivatives	$ 0	$ 0