Commitments And Contingencies (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) partnership center	Dec. 31, 2016 USD ($) partnership center	Dec. 31, 2015 USD ($)
Long-term Purchase Commitment [Line Items]
Operating leases, rental expense	$ 2,900	$ 3,000	$ 3,200
Number of real estate properties | center	2	2
Capital leases, total assets	$ 16,800	$ 16,800
Capital leases, accumulated depreciation	15,500	$ 14,200
Capital leases, future minimum payments	31,226
Capital leases, future minimum payments, interest	10,200
Capital leases, future minimum payments, present value of net minimum payments	$ 21,000
Minimum [Member]
Long-term Purchase Commitment [Line Items]
Operating leases, term range, in years	1 year
Maximum [Member]
Long-term Purchase Commitment [Line Items]
Operating leases, term range, in years	25 years
Capital Additions [Member]
Long-term Purchase Commitment [Line Items]
Purchase contract, commitment	$ 114,700
Capital Additions [Member] | Minimum [Member]
Long-term Purchase Commitment [Line Items]
Purchase contract, period (in months)	12 months
Capital Additions [Member] | Maximum [Member]
Long-term Purchase Commitment [Line Items]
Purchase contract, period (in months)	36 months
DownREIT [Member]
Long-term Purchase Commitment [Line Items]
Number of real estate joint ventures | partnership	2	2
Aggregate redemption value	$ 47,000	$ 52,000