Fair Value Measurement - Cash and Cash Equivalents (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Assets, Fair Value Disclosure
Interest rate cash flow hedge, assets			$ 473
Total Assets, Fair Value	$ 23,615	$ 90,438	62,753
Liabilities, Fair Value Disclosure
Interest rate cash flow hedge, liabilities	4,985	1,098
Total liabilities	4,985	1,098
Money Market Funds [Member]
Assets, Fair Value Disclosure
Cash equivalents	17,790	84,613	56,534
Restricted Cash [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	5,825	5,825	5,746
Level 1 [Member]
Assets, Fair Value Disclosure
Interest rate cash flow hedge, assets			0
Total Assets, Fair Value	23,615	90,438	62,280
Liabilities, Fair Value Disclosure
Interest rate cash flow hedge, liabilities	0	0
Total liabilities	0	0
Level 1 [Member] | Money Market Funds [Member]
Assets, Fair Value Disclosure
Cash equivalents	17,790	84,613	56,534
Level 1 [Member] | Restricted Cash [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	5,825	5,825	5,746
Level 2 [Member]
Assets, Fair Value Disclosure
Interest rate cash flow hedge, assets			473
Total Assets, Fair Value	0	0	473
Liabilities, Fair Value Disclosure
Interest rate cash flow hedge, liabilities	4,985	1,098
Total liabilities	4,985	1,098
Level 2 [Member] | Money Market Funds [Member]
Assets, Fair Value Disclosure
Cash equivalents	0	0	0
Level 2 [Member] | Restricted Cash [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	0	0	0
Level 3 [Member]
Assets, Fair Value Disclosure
Interest rate cash flow hedge, assets			0
Total Assets, Fair Value	0	0	0
Liabilities, Fair Value Disclosure
Interest rate cash flow hedge, liabilities	0	0
Total liabilities	0	0
Level 3 [Member] | Money Market Funds [Member]
Assets, Fair Value Disclosure
Cash equivalents	0	0	0
Level 3 [Member] | Restricted Cash [Member]
Assets, Fair Value Disclosure
Other noncurrent assets	$ 0	$ 0	$ 0