Aggregate dividends declared during the period for each share of common stock outstanding.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Cash received from principal payments made on loans related to operating activities.

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Subsequent Events (Details) - USD ($)						3 Months Ended	12 Months Ended
Subsequent Events (Details) - USD ($)	Feb. 05, 2018	Jan. 26, 2018	Jan. 22, 2018	Nov. 12, 2015	Oct. 30, 2015	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent events
Dividends declared for common stock (in dollars per share)				$ 2.00	$ 2.00		$ 2.87	$ 2.75	$ 6.63
Subsequent event
Subsequent events
Dividends declared for common stock (in dollars per share)		$ 0.74
Subsequent event \| Joint venture \| Fashion District Philadelphia
Subsequent events
Variable interest rate spread (as a percent)			2.00%
Subsequent event \| Joint venture \| Fashion District Philadelphia \| Term loan
Subsequent events
Face amount of debt			$ 250,000,000
Proceeds from Loans			$ 150,000,000
Subsequent event \| Joint venture \| Fashion District Philadelphia \| Term loan \| Forecast
Subsequent events
Proceeds from Loans						$ 100,000,000
Subsequent event \| Joint venture \| Broadway Plaza
Subsequent events
Face amount of debt	$ 450,000,000
Interest rate on debt (as a percent)	4.18%
Debt term (years)	12 years