Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Assets
Total assets	$ 126,570	$ 248,275
Liabilities
Accrued other current liabilities	433	1,331
Total liabilities	20,446	14,690
Cash flow hedge gain to be reclassified within 12 months	800
Money market funds
Assets
Cash equivalents:	126,564	143,843
Short-term investments
Assets
Short-term investments		104,350
Natural gas fixed price forward contracts
Liabilities
Derivatives:	8,465	9,729
Interest rate swap agreements
Assets
Interest rate swap agreements	6	82
Liabilities
Derivatives:	11,548	3,630
Level 1
Assets
Total assets	126,564	248,193
Liabilities
Accrued other current liabilities	433	1,331
Total liabilities	433	1,331
Level 1 | Money market funds
Assets
Cash equivalents:	126,564	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	6	82
Liabilities
Accrued other current liabilities	0	0
Total liabilities	11,548	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	6	82
Liabilities
Derivatives:	11,548	3,630
Level 3
Assets
Total assets	0	0
Liabilities
Accrued other current liabilities	0	0
Total liabilities	8,465	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:	8,465	9,729
Level 3 | Interest rate swap agreements
Assets
Interest rate swap agreements	0	0
Liabilities
Derivatives:	$ 0	$ 0