Commitments and Contingencies - Installment Contracts (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Long-term Purchase Commitment [Line Items]
Current reflected in other current liabilities	$ 2,978	$ 2,157
Installment Contracts
Long-term Purchase Commitment [Line Items]
2018	3,544
2019	1,988
2020	1,280
2021	1,243
2022	1,020
Thereafter	5,746
Total	14,821
Less amounts representing interest	(2,861)
Present value	11,960
Current reflected in other current liabilities	2,978
Noncurrent reflected in other liabilities	$ 8,982