Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

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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No definition available.

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- Definition

The cash outflow to repay long-term debt that is wholly or partially secured by collateral. Excludes repayments of tax exempt secured debt.

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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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Real Estate Investments - Purchase Price Allocation (Parenthetical) (Details) - USD ($) $ in Thousands					12 Months Ended
	Dec. 11, 2018	Jan. 05, 2018	Oct. 13, 2017	Jul. 28, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Outstanding principal balances on mortgage debt assumed					$ 320,869	$ 317,216
Repayments of mortgage debt assumed					$ 122,180	$ 4,931	$ 357,151
Mortgage Debt Assumed - at Fair Value [Member]
Business Acquisition [Line Items]
Outstanding principal balances on mortgage debt assumed	$ 115,500
Repayments of mortgage debt assumed	$ 115,500
Four Tower Bridge [Member] \| Mortgage Debt Assumed - at Fair Value [Member]
Business Acquisition [Line Items]
Outstanding principal balances on mortgage debt assumed		$ 9,700
Four Tower Bridge [Member] \| Intangible Assets Acquired [Member]
Business Acquisition [Line Items]
Weighted average amortization period		4 years 1 month 6 days
Four Tower Bridge [Member] \| Below Market Lease [Member]
Business Acquisition [Line Items]
Weighted average amortization period		4 years 9 months 18 days
Bulletin Building [Member] \| Intangible Assets Acquired [Member]
Business Acquisition [Line Items]
Weighted average amortization period			7 years 10 months 24 days
Bulletin Building [Member] \| Below Market Lease [Member]
Business Acquisition [Line Items]
Weighted average amortization period			7 years
3000 Market Street [Member] \| Intangible Assets Acquired [Member]
Business Acquisition [Line Items]
Weighted average amortization period				5 years 10 months 24 days
3000 Market Street [Member] \| Below Market Lease [Member]
Business Acquisition [Line Items]
Weighted average amortization period				6 years