Mortgage Notes Payable, Credit Facilities and Senior Notes - Combined Aggregate Principal Maturities of Mortgage Notes Payable (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Principal Repayments
Long-term Debt	$ 298,816	$ 299,191
Mortgage Premium
Total	1,921	1,037
Total
Total mortgage notes payable	107,915	$ 71,303
Mortgage Notes Payable
Principal Repayments
2018	18,900
2019	0
2020	0
2021	0
2022	23,130
Thereafter	58,787
Long-term Debt	100,817
Scheduled Amortization
2018	712
2019	550
2020	577
2021	717
2022	1,003
Thereafter	1,944
Total	5,503
Mortgage Premium
2017	493
2018	360
2019	360
2020	360
2021	223
Thereafter	125
Total	1,921
Total
2018	20,105
2019	910
2020	937
2021	1,077
2022	24,356
Thereafter	60,856
Total mortgage notes payable	$ 108,241