Debt - Unsecured Term Loan Facilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2017
Long-term Debt, Total	$ 775,200
Unsecured Debt [Member]
Total Principal	258,353	$ 258,543
Unamortized debt issuance costs	(2,109)	(2,124)
Long-term Debt, Total	256,244	256,419
Unsecured Debt [Member] \| 2019 Term Loan [Member]
Total Principal	18,353	18,543
Unsecured Debt [Member] \| 2023 Term Loan [Member]
Total Principal	40,000	40,000
Unsecured Debt [Member] \| 2024 Term Loans [Member]
Total Principal	100,000	100,000
Unsecured Debt [Member] \| 2026 Term Loan [Member]
Total Principal	$ 100,000	$ 100,000

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant