Fair Value Measurement - Cash and Cash Equivalents (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Assets, Fair Value Disclosure
Total Assets, Fair Value	$ 41,984	$ 90,438	$ 7,056
Other current liabilities	0	0
Interest rate cash flow hedge	2,530	1,098
Total liabilities	2,530	1,098
Other current assets			0
Interest rate cash flow hedge			2,165
Level 1 [Member]
Assets, Fair Value Disclosure
Total Assets, Fair Value	41,984	90,438	4,891
Other current liabilities	0	0
Interest rate cash flow hedge	0	0
Total liabilities	0	0
Other current assets			0
Interest rate cash flow hedge			0
Level 2 [Member]
Assets, Fair Value Disclosure
Total Assets, Fair Value	0	0	2,165
Other current liabilities	0	0
Interest rate cash flow hedge	2,530	1,098
Total liabilities	2,530	1,098
Other current assets			0
Interest rate cash flow hedge			2,165
Level 3 [Member]
Assets, Fair Value Disclosure
Total Assets, Fair Value	0	0	0
Other current liabilities	0	0
Interest rate cash flow hedge	0	0
Total liabilities	0	0
Other current assets			0
Interest rate cash flow hedge			0
Money Market Funds [Member]
Assets, Fair Value Disclosure
Cash equivalents	36,159	84,613	4,891
Money Market Funds [Member] | Level 1 [Member]
Assets, Fair Value Disclosure
Cash equivalents	36,159	84,613	4,891
Money Market Funds [Member] | Level 2 [Member]
Assets, Fair Value Disclosure
Cash equivalents	0	0	0
Money Market Funds [Member] | Level 3 [Member]
Assets, Fair Value Disclosure
Cash equivalents	0	0	$ 0
Restricted Cash [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	5,825	5,825
Restricted Cash [Member] | Level 1 [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	5,825	5,825
Restricted Cash [Member] | Level 2 [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	0	0
Restricted Cash [Member] | Level 3 [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	$ 0	$ 0