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Mortgage Notes Payable, Revolving Credit Facility, and Term Loan Facility - Mortgage Notes Payable (Details) $ in Thousands |  | 12 Months Ended |
--- | --- | --- | ---
Oct. 27, 2017 | Dec. 31, 2018 USD ($) property | Dec. 31, 2017 USD ($) property | Dec. 31, 2016 USD ($) | Sep. 30, 2018
Debt Instrument [Line Items] |  |  |  |  |
Debt repaid |  | $ 27,850 | $ 60,080 | $ 88,899 |
Weighted Average Interest Rate on Fixed Rate Debt Extended |  | 4.59% |  |  |
Number of properties acquired | property |  | 5 | 7 |  |
Fixed rate mortgage loans |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt repaid |  | $ 9,444 |  |  |
Stated interest rate (as a percent) |  | 5.75% |  |  |
Variable rate mortgage loans |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt repaid |  | $ 6,738 |  |  |
New fixed rate mortgage notes payable |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Aggregate Fixed Rate Debt Issued or Assumed |  | $ 21,043 |  |  |
Number of properties acquired in connection with fixed rate | property |  | 2 |  |  |
Number of properties acquired | property |  | 5 |  |  |
Stated interest rate (as a percent) |  | 4.76% |  |  |
Fixed rate mortgage loans debt extended |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Aggregate Fixed Rate Debt Extended |  | $ 3,598 |  |  |
Weighted Average Interest Rate on Fixed Rate Debt Extended |  | 4.92% |  |  |
Debt instrument term |  | 3 years |  |  |
Variable rate mortgage loans debt extended |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Aggregate Fixed Rate Debt Issued or Assumed |  | $ 24,832 |  |  |
Debt instrument term |  | 1 year 6 months 8 days |  |  |
LIBOR |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Interest Rate on Variable Rate Debt Repaid | 2.75% |  |  |  |
LIBOR | Variable rate mortgage loans | Weighted Average |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Interest Rate on Variable Rate Debt Repaid |  | 2.25% |  |  |
LIBOR | Variable rate mortgage loans debt extended | Weighted Average |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Interest Rate on Variable Rate Debt Repaid |  | 2.29027% |  |  |
Interest rate swaps | New fixed rate mortgage notes payable |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Number of interest rate swaps | property |  | 2 |  |  |
Interest rate swap one | New fixed rate mortgage notes payable |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Stated interest rate (as a percent) |  |  |  |  | 4.58%
Interest rate swap two | New fixed rate mortgage notes payable |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Stated interest rate (as a percent) |  | 5.32% |  |  |
Detroit, Michigan | Interest rate swaps | New fixed rate mortgage notes payable |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Stated interest rate (as a percent) |  | 4.625% |  |  |

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