Recent Transactions (Rockpoint Transaction) (Narrative) (Details) - USD ($) $ in Thousands			12 Months Ended
	Mar. 10, 2017	Feb. 27, 2017	Dec. 31, 2018	Dec. 31, 2017
Preferred Units [Line Items]
Assets			$ 1,115,458	$ 1,139,322
Mortgages and loans payable			686,797	689,412
Other liabilities			67,072	80,746
RRLP [Member]
Preferred Units [Line Items]
Assets			2,300,000	1,900,000
Mortgages and loans payable			1,100,000	769,700
Other liabilities			57,000	$ 95,900
Investment Agreement [Member] | Rockpoint [Member]
Preferred Units [Line Items]
Incremental closing payments, Limited Partnership interest	$ 150,000		105,000
Investment Agreement [Member] | Rockpoint [Member] | Minimum [Member]
Preferred Units [Line Items]
Incremental closing payments, Limited Partnership interest			$ 10,000
Investment Agreement [Member] | Rockpoint [Member] | Maximum [Member]
Preferred Units [Line Items]
Contributed amount to obtain equity units		$ 300,000