Notes Payable, Revolving Credit Facility, Interest and Amortization of Deferred Debt Costs - Scheduled Maturities of Debt, Including Scheduled Principal Amortization (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Balloon Payments
October 1 through December 31, 2018	$ 0
2019	60,794
2020	61,163
2021	11,012
2022	113,502
2023	84,225
Thereafter	430,259
Principal amount	760,955
Scheduled Principal Amortization
October 1 through December 31, 2018	7,688
2019	29,298
2020	26,772
2021	26,486
2022	26,990
2023	27,290
Thereafter	117,422
Principal amount	261,946
Total
October 1 through December 31, 2018	7,688
2019	90,092
2020	87,935
2021	37,498
2022	140,492
2023	111,515
Thereafter	547,681
Principal amount	1,022,901	$ 965,500
Unamortized deferred debt costs	9,989	6,900
Net	1,012,912
Debt Instrument [Line Items]
Outstanding line of credit	75,200	60,734
Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Outstanding line of credit	$ 77,000	$ 61,000