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Mortgage Notes Payable and Credit Facility - Mortgages Notes Payable (Detail) $ in Thousands | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2019 USD ($) mortgage property | Sep. 30, 2018 USD ($) property | Dec. 31, 2018 USD ($) | Dec. 27, 2018 property
Debt Instrument [Line Items] |  |  |  |
Aggregate Fixed Rate Debt Repaid | $ 48,116 | $ 24,888 |  |
Number of properties acquired | property | 9 | 2 |  |
Variable rate mortgage loans |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Aggregate Fixed Rate Debt Repaid |  |  | $ 13,600 |
Variable rate mortgage loans | LIBOR | Weighted average |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Spread on LIBOR (as percent) |  |  | 2.47% |
Variable rate mortgage loans | LIBOR | Weighted average | Property maturing February 8, 2029 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Spread on LIBOR (as percent) | 1.80% |  |  |
Fixed rate mortgage loans |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Aggregate Fixed Rate Debt Repaid | $ 25,042 |  |  |
Stated interest rate (as percent) | 4.18% |  |  |
Fixed rate mortgage loans | LIBOR | Weighted average | Property maturing February 8, 2029 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Spread on LIBOR (as percent) | 1.80% |  |  |
New fixed rate mortgage notes payable |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate (as percent) | 3.95% |  |  |
Aggregate Fixed Rate Debt Issued | $ 41,140 |  |  |
New fixed rate mortgage notes payable | Property maturing February 8, 2029 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate (as percent) | 4.70% |  |  |
Term of debt | 7 years |  |  |
Aggregate Fixed Rate Debt Issued | $ 10,600 |  |  |
Number of properties acquired | property |  |  |  | 1
New fixed rate mortgage notes payable | Property maturing June 1, 2024 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate (as percent) | 3.42% |  |  |
Aggregate Fixed Rate Debt Issued | $ 21,600 |  |  |
Number of properties acquired | property |  |  |  | 1
New fixed rate mortgage notes payable | Property maturing June 18, 2024 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Stated interest rate (as percent) | 4.35% |  |  |
Aggregate Fixed Rate Debt Issued | $ 8,900 |  |  |
New fixed rate mortgage notes payable | Treasury Rate | Property maturing February 8, 2029 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Term of debt | 3 years |  |  |
Mortgages | Variable rate mortgage loans debt extended |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Term of debt | 2 years 4 months 10 days |  |  |
Aggregate Variable Rate Debt Extended | $ 12,561 |  |  |
Number of Long Term Mortgages Maturities Extended | mortgage | 2 |  |  |
Number of Properties Maturity Extended to Collateralized Mortgage Notes Payable | property | 4 |  |  |
Mortgages | Variable rate mortgage loans debt extended | LIBOR | Weighted average |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Spread on LIBOR (as percent) | 2.51117% |  |  |

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