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Mortgage Notes Payable, Revolving Credit Facility, and Term Loan Facility - Additional Information (Detail) |  |  |  | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 27, 2017 | Jun. 22, 2017 USD ($) | Oct. 05, 2015 USD ($) | Dec. 31, 2017 USD ($) property mortgage | Jul. 31, 2017 | Jul. 07, 2017 | Dec. 31, 2016 USD ($) property | Aug. 31, 2013 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Number of properties collateralized in mortgage notes payable | property |  |  |  | 96 |  |  |  |
Net book value of collateralized mortgage properties |  |  |  | $ 5,900,000 |  |  |  |
Number of properties acquired in connection with fixed rate | property |  |  |  | 5 |  |  |  |
Number of properties acquired | property |  |  |  | 7 |  |  | 3 |
Derivative asset |  |  |  | $ 504,000 |  |  | $ 101,000 |
Fair value of mortgage notes payable outstanding |  |  |  | 455,100,000 |  |  |  |
Long-term debt |  |  |  | 542,627,000 |  |  | 509,395,000 |
Line of credit facility, maximum borrowing capacity |  |  |  | 75,000,000.0 |  |  |  | $ 25,000,000.0
Debt instrument, fee | 0.9 |  |  |  |  |  |  |
Borrowings under revolving credit facility, net |  |  |  | $ 20,715,000 |  |  | 39,225,000 |
Line of credit at an interest rate (as percent) |  |  |  | 3.28% |  |  |  |
Letters of credit, outstanding |  |  |  | $ 1,000,000 |  |  |  |
Weighted average interest rate on debt outstanding (as percent) |  |  |  | 4.42% |  |  |  |
Mortgage Notes Payable |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Number of mortgage notes payable | mortgage |  |  |  | 45 |  |  |  |
Number of properties collateralized in mortgage notes payable | property |  |  |  | 67 |  |  |  |
Net book value of collateralized mortgage properties |  |  |  | $ 654,900,000 |  |  |  |
Collateral |  |  |  | $ 11,700,000 |  |  |  |
Collateral (as percent) |  |  |  | 2.60% |  |  |  |
Number of mortgages repaid | mortgage |  |  |  | 4 |  |  |  |
Number of properties collateralized | property |  |  |  | 10 |  |  |  |
Number of long-term mortgages issued | property |  |  |  | 4 |  |  |  |
Number of properties to issue collateralized mortgage notes payable | property |  |  |  | 7 |  |  |  |
Long-term debt |  |  |  | $ 447,380,000 |  |  | $ 445,278,000 |
Carrying value of debt |  |  |  | $ 452,491,000 |  |  |  |
Weighted average interest rate on debt outstanding (as percent) |  |  |  | 4.56% |  |  |  |
New Fixed Rate Mortgage Notes Payable |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Noncash or part noncash acquisition, debt assumed |  |  |  | $ 54,900,000 |  |  |  |
Number of properties acquired | property |  |  |  | 7 |  |  |  |
Weighted average interest rate on debt outstanding (as percent) |  |  |  | 3.78% |  |  |  |
New Variable Rate Mortgage Notes Payable |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  | 2.75% |  |  |  |
Additional Advance On New Variable Rate Mortgage Notes Payable [Member] |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  | 2.50% |  |  |  |
Minimum | Philadelphia, PA | New Fixed Rate Mortgage Notes Payable |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  |  | 3.75% |  |
Maximum | Maitland, Florida | New Fixed Rate Mortgage Notes Payable |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  | 3.89% |  |  |
Interest Rate Swap |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Number of properties acquired in connection with interest rate swap | property |  |  |  | 1 |  |  |  |
Interest rate swap assumed |  | 1.80% |  |  |  |  |  |
Interest Rate Swap | New Fixed Rate Mortgage Notes Payable |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  | 3.545% |  |  |  |
Interest Rate Swap | Conshohocken, Pennsylvania |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Noncash or part noncash acquisition, debt assumed |  | $ 11,200,000 |  |  |  |  |  |
Derivative asset |  | $ 40,000 |  | $ 300,000 |  |  |  |
London Interbank Offered Rate (LIBOR) |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Libor rate (as percent) | 2.75% |  |  | 1.56% |  |  |  |
Five Year Term Loan Facility |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt instrument term | 5 years |  |  |  |  |  |  |
Revolving Credit Facility |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Debt instrument term | 4 years |  |  |  |  |  |  |
Line of Credit |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  | $ 85,000,000.0 |  |  |  |  |
Borrowings under revolving credit facility, net |  |  |  | $ 96,400,000 |  |  |  |
Line of credit facility, maximum additional amount drawn |  |  |  | $ 43,200,000 |  |  |  |
Line of Credit | Five Year Term Loan Facility |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  | $ 25,000,000.0 |  |  |  |  |
Debt instrument term |  |  | 5 years |  |  |  |  |
Letter of Credit |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Weighted average interest rate on debt outstanding (as percent) |  |  |  | 1.75% |  |  |  |

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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313 + Details
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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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