Debt - Unsecured Term Loan Facilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Total Principal	$ 931,900
Long-term Debt, Total	931,867
Unsecured Debt [Member]
Total Principal	240,000	$ 258,543
Unamortized debt issuance costs	(2,016)	(2,124)
Long-term Debt, Total	237,984	256,419
Unsecured Debt [Member] | 2019 Term Loan [Member]
Total Principal		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