Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets
Total assets	$ 217,285	$ 248,275
Liabilities
Accrued other current liabilities	1,723	1,331
Total liabilities	16,238	14,690
Money market funds
Assets
Cash equivalents:	217,258	143,843
Short-term investments
Assets
Short-term investments		104,350
Natural gas fixed price forward contracts
Liabilities
Derivatives:	8,796	9,729
Interest rate swap agreements
Assets
Interest rate swap agreements	27	82
Liabilities
Derivatives:	5,719	3,630
Level 1
Assets
Total assets	217,258	248,193
Liabilities
Accrued other current liabilities	1,723	1,331
Total liabilities	1,723	1,331
Level 1 | Money market funds
Assets
Cash equivalents:	217,258	143,843
Level 1 | Short-term investments
Assets
Short-term investments		104,350
Level 1 | Natural gas fixed price forward contracts
Liabilities
Derivatives:	0	0
Level 1 | Interest rate swap agreements
Assets
Interest rate swap agreements	0	0
Liabilities
Derivatives:	0	0
Level 2
Assets
Total assets	27	82
Liabilities
Accrued other current liabilities	0	0
Total liabilities	5,719	3,630
Level 2 | Money market funds
Assets
Cash equivalents:	0	0
Level 2 | Short-term investments
Assets
Short-term investments		0
Level 2 | Natural gas fixed price forward contracts
Liabilities
Derivatives:	0	0
Level 2 | Interest rate swap agreements
Assets
Interest rate swap agreements	27	82
Liabilities
Derivatives:	5,719	3,630
Level 3
Assets
Total assets	0	0
Liabilities
Accrued other current liabilities	0	0
Total liabilities	8,796	9,729
Level 3 | Money market funds
Assets
Cash equivalents:	0	0
Level 3 | Short-term investments
Assets
Short-term investments		0
Level 3 | Natural gas fixed price forward contracts
Liabilities
Derivatives:	8,796	9,729
Level 3 | Interest rate swap agreements
Assets
Interest rate swap agreements	0	0
Liabilities
Derivatives:	$ 0	$ 0