Notes and Bonds Payable - Future Maturities (Details) $ in Thousands	Dec. 31, 2018 USD ($) mortgage_loan	Dec. 31, 2017 USD ($)
Long Term Debt Maturities [Line Items]
Number of outstanding mortgage notes | mortgage_loan	17
Excluded debt issuance costs	$ 2,200
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	1,355,115
Net Accretion/Amortization	(2,597)
Debt Issuance Costs (2)	(6,534)
Notes and bonds payable	$ 1,345,984	$ 1,283,880
%	100.00%
2019 [Member]
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	$ 13,857
Net Accretion/Amortization	(250)
Debt Issuance Costs (2)	(1,067)
Notes and bonds payable	$ 12,540
%	0.90%
2020 [Member]
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	$ 285,064
Net Accretion/Amortization	(392)
Debt Issuance Costs (2)	(1,061)
Notes and bonds payable	$ 283,611
%	21.10%
2021 [Member]
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	$ 17,593
Net Accretion/Amortization	(326)
Debt Issuance Costs (2)	(1,046)
Notes and bonds payable	$ 16,221
%	1.20%
2022 [Member]
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	$ 162,977
Net Accretion/Amortization	(336)
Debt Issuance Costs (2)	(1,058)
Notes and bonds payable	$ 161,583
%	12.00%
2023 [Member]
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	$ 280,230
Net Accretion/Amortization	(183)
Debt Issuance Costs (2)	(697)
Notes and bonds payable	$ 279,350
%	20.80%
2024 and thereafter [Member]
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	$ 595,394
Net Accretion/Amortization	(1,110)
Debt Issuance Costs (2)	(1,605)
Notes and bonds payable	$ 592,679
%	44.00%