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Notes Payable, Revolving Credit Facility, Interest and Amortization of Deferred Debt Costs - Narrative (Details) - USD ($) |  |  |  |  |  | 3 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2018 | Nov. 20, 2017 | Sep. 01, 2017 | Aug. 14, 2017 | Jan. 18, 2017 | Dec. 31, 2021 | Jan. 31, 2018 | Jan. 26, 2018 | Dec. 31, 2017
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Outstanding debt | $ 975,528,000 |  |  |  |  |  |  |  |
Debt outstanding with fixed-rate | 875,500,000 |  |  |  |  |  |  |  | $ 890,400,000
Debt outstanding with variable-rate | 100,000,000 |  |  |  |  |  |  |  | 75,100,000
Outstanding line of credit | 23,065,000 |  |  |  |  |  |  |  | 60,734,000
Carrying value of properties collateralizing mortgage notes | 969,000,000 |  |  |  |  |  |  |  | 1,000,000,000
Final payment amount | 706,003,000 |  |  |  |  |  |  |  |
Principal amount | 269,525,000 |  |  |  |  |  |  |  |
Construction loan | 869,272,000 |  |  |  |  |  |  |  | 897,888,000
Total outstanding debt | 975,500,000 |  |  |  |  |  |  |  | 965,500,000
Unsecured Revolving Credit Facility |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Outstanding line of credit | 25,000,000 |  |  |  |  |  |  |  | $ 61,000,000
Line of credit facility, remaining borrowing capacity | 208,900,000 |  |  |  |  |  |  |  |
Letter of Credit |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity | 185,000 |  |  |  |  |  |  |  |
New Facility | Line of Credit |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  |  |  |  |  | $ 400,000,000 |
New Facility | Term Loan | Line of Credit |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Outstanding line of credit | $ 75,000,000 |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  |  |  |  |  | 75,000,000 |
New Facility | Term Loan | LIBOR |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate spread on LIBOR (percentage) | 1.30% |  |  |  |  |  |  |  |
New Facility | Term Loan | LIBOR | Minimum |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate spread on LIBOR (percentage) | 1.30% |  |  |  |  |  |  |  |
New Facility | Term Loan | LIBOR | Maximum |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate spread on LIBOR (percentage) | 1.90% |  |  |  |  |  |  |  |
New Facility | Revolving Line | Line of Credit |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Line of credit facility, maximum borrowing capacity |  |  |  |  |  |  |  | $ 325,000,000 |
New Facility | Revolving Line | LIBOR |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate spread on LIBOR (percentage) | 1.35% |  |  |  |  |  |  |  |
New Facility | Revolving Line | LIBOR | Minimum |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate spread on LIBOR (percentage) | 1.35% |  |  |  |  |  |  |  |
New Facility | Revolving Line | LIBOR | Maximum |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate spread on LIBOR (percentage) | 1.95% |  |  |  |  |  |  |  |
Burtonsville Fixed Rate Mortgage |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, term |  |  |  |  | 15 years |  |  |  |
Construction-to-permanent loan |  |  |  |  | $ 40,000,000 |  | $ 40,000,000.0 |  |
Interest rate, stated percentage |  |  |  |  | 3.39% |  |  |  |
Periodic payment |  |  |  |  | $ 197,900 |  |  |  |
Amortization period |  |  |  |  | 25 years |  |  |  |
Final payment amount |  |  |  |  | $ 20,300,000 |  |  |  |
Glebe Road Construction Loan |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Construction-to-permanent loan |  |  |  | $ 157,000,000.0 |  |  |  |  |
Interest rate, stated percentage |  |  |  | 4.67% |  |  |  |  |
Amortization period |  |  |  | 25 years |  |  |  |  |
Park Van Ness Loan |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest rate, stated percentage |  |  | 4.88% |  |  |  |  |  |
Periodic payment |  |  | $ 413,460 |  |  |  |  |  |
Amortization period |  |  | 25 years |  |  |  |  |  |
Final payment amount |  |  | $ 39,600,000 |  |  |  |  |  |
Guarantor obligations, maximum exposure | $ 53,700,000 |  |  |  |  |  |  |  |
Construction loan | 70,500,000 |  |  |  |  |  |  |  |
Washington Square 2017 Mortgage | 3.75% due December 2032 |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, term |  | 15 years |  |  |  |  |  |  |
Construction-to-permanent loan |  | $ 60,000,000 |  |  |  |  |  |  |
Interest rate, stated percentage |  | 3.75% |  |  |  |  |  |  |
Periodic payment |  | $ 308,500 |  |  |  |  |  |  |
Amortization period |  | 25 years |  |  |  |  |  |  |
Final payment amount |  | $ 31,100,000 |  |  |  |  |  |  |
Kentlands Square II Bank Loan | Fixed Rate Mortgage Notes Payable |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Principal amount | 35,900,000 |  |  |  |  |  |  |  |
Guarantor obligations, maximum exposure | $ 9,200,000 |  |  |  |  |  |  |  |
Forecast | Glebe Road Construction Loan |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Periodic payment |  |  |  |  |  | $ 887,900 |  |  |
Washington Square |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Principal amount |  | $ 28,100,000 |  |  |  |  |  |  |

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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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