Notes and Bonds Payable - Future Maturities (Details) $ in Thousands	Dec. 31, 2017 USD ($) mortgage_note_payable	Dec. 31, 2016 USD ($)
Long Term Debt Maturities [Line Items]
Number of outstanding mortgage notes | mortgage_note_payable	18
Excluded debt issuance costs	$ 3,500
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	1,293,916
Net Accretion/Amortization	(2,548)
Debt Issuance Costs (2)	(7,488)
Notes and bonds payable	$ 1,283,880	$ 1,264,370
%	100.00%
2018 [Member]
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	$ 10,605
Net Accretion/Amortization	(18)
Debt Issuance Costs (2)	(1,051)
Notes and bonds payable	$ 9,536
%	0.70%
2019 [Member]
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	$ 22,711
Net Accretion/Amortization	(224)
Debt Issuance Costs (2)	(1,044)
Notes and bonds payable	$ 21,443
%	1.70%
2020 [Member]
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	$ 211,803
Net Accretion/Amortization	(382)
Debt Issuance Costs (2)	(1,039)
Notes and bonds payable	$ 210,382
%	16.40%
2021 [Member]
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	$ 17,321
Net Accretion/Amortization	(312)
Debt Issuance Costs (2)	(1,025)
Notes and bonds payable	$ 15,984
%	1.20%
2022 [Member]
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	$ 162,692
Net Accretion/Amortization	(328)
Debt Issuance Costs (2)	(1,037)
Notes and bonds payable	$ 161,327
%	12.60%
2023 and thereafter [Member]
Future contractual maturities of the Company's notes and bonds payable
Principal Maturities	$ 868,784
Net Accretion/Amortization	(1,284)
Debt Issuance Costs (2)	(2,292)
Notes and bonds payable	$ 865,208
%	67.40%