Represents the number of properties secured by mortgage notes.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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.

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The portion of losses on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness.

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Gross amount of debt extinguished.

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of real estate investment property, net of accumulated depreciation, which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; and (7) other real estate investments.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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Indebtedness - Narrative (Details)									1 Months Ended			11 Months Ended	12 Months Ended
Indebtedness - Narrative (Details)	Dec. 26, 2018 USD ($)	May 04, 2018 USD ($)	Mar. 07, 2018 USD ($)	Jul. 15, 2017 USD ($)	Dec. 15, 2016 USD ($)	Nov. 10, 2016 USD ($)	Feb. 16, 2016 USD ($)	Jan. 29, 2015 USD ($)	Dec. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($)	Apr. 30, 2017 USD ($)	Dec. 31, 2015	Dec. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Loss on early extinguishment of debt													$ 7,122,000	$ 493,000	$ 2,680,000
Real estate properties, net									$ 763,674,000	$ 1,296,893,000			763,674,000	1,296,893,000
Mortgage notes payable, net									26,482,000	32,594,000			26,482,000	32,594,000
Designated as hedging instrument \| Cash flow hedging
Debt Instrument [Line Items]
Expense included in interest and other income													$ 293,000	0	241,000
Mortgage notes \| 97 Newberry Road
Debt Instrument [Line Items]
Amount of debt, redeemed, defeased, or prepaid									4,900,000
Loss on early extinguishment of debt									$ 600,000
Mortgage notes \| 33 Stiles Lane
Debt Instrument [Line Items]
Amount of debt, redeemed, defeased, or prepaid										$ 2,000,000
Loss on early extinguishment of debt														200,000
Mortgage notes \| Parkshore Plaza
Debt Instrument [Line Items]
Amount of debt, redeemed, defeased, or prepaid											$ 41,300,000
Loss on early extinguishment of debt														$ 100,000
Mortgage notes \| 1735 Market Street
Debt Instrument [Line Items]
Amount of debt, redeemed, defeased, or prepaid						$ 167,800,000
Loss on early extinguishment of debt															2,400,000
Mortgage notes \| 1735 Market Street \| Designated as hedging instrument \| Interest rate swap contracts \| Cash flow hedging
Debt Instrument [Line Items]
Expense included in interest and other income															$ 200,000
Loan facility, 5-year term
Debt Instrument [Line Items]
Interest rate stated percentage									0.00%				0.00%
Loan facility, 5-year term \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis points on variable rate												0.90%
Loan facility, 5-year term \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis points on variable rate												1.80%
Loan facility, 7-year term
Debt Instrument [Line Items]
Interest rate stated percentage									0.00%				0.00%
Loan facility, 7-year term \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis points on variable rate												1.40%
Loan facility, 7-year term \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis points on variable rate												2.35%
5.75% Senior Unsecured Notes due 2024
Debt Instrument [Line Items]
Amount of debt, redeemed, defeased, or prepaid			$ 175,000,000
Loss on early extinguishment of debt			$ 4,900,000
Interest rate stated percentage			5.75%
6.65% Senior Unsecured Notes due 2018
Debt Instrument [Line Items]
Amount of debt, redeemed, defeased, or prepaid				$ 250,000,000
Loss on early extinguishment of debt				$ 200,000
Interest rate stated percentage				6.65%
6.25% Senior Unsecured Notes due 2017
Debt Instrument [Line Items]
Amount of debt, redeemed, defeased, or prepaid					$ 250,000,000
Loss on early extinguishment of debt					$ 100,000
Interest rate stated percentage					6.25%
6.25% Senior Unsecured Notes due 2016
Debt Instrument [Line Items]
Amount of debt, redeemed, defeased, or prepaid							$ 139,100,000
Loss on early extinguishment of debt							$ 100,000
Interest rate stated percentage							6.25%
Mortgage notes
Debt Instrument [Line Items]
Number of real estate properties secured by mortgage \| property									1				1
Real estate properties, net									$ 44,300,000				$ 44,300,000
Mortgage notes payable, net									$ 26,500,000				$ 26,500,000
Revolving credit facility \| Minimum
Debt Instrument [Line Items]
Commitment fee percentage												0.125%
Revolving credit facility \| Maximum
Debt Instrument [Line Items]
Commitment fee percentage												0.30%
Revolving credit facility \| Unsecured revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity								$ 750,000,000.0
Revolving credit facility \| Unsecured revolving credit facility \| Line of credit
Debt Instrument [Line Items]
Loss on early extinguishment of debt	$ 200,000
Revolving credit facility \| Loan facility, 5-year term
Debt Instrument [Line Items]
Maximum borrowing capacity								$ 200,000,000.0
Debt instrument, term								5 years
Revolving credit facility \| Loan facility, 7-year term
Debt Instrument [Line Items]
Maximum borrowing capacity								$ 200,000,000.0
Debt instrument, term								7 years
Revolving credit facility \| Loan facility, 5-year term and 7-year term
Debt Instrument [Line Items]
Amount of debt, redeemed, defeased, or prepaid		$ 400,000,000
Loss on early extinguishment of debt		$ 1,500,000