Commitments and Contingencies (Narrative) (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2021 USD ($) Partnership	Dec. 31, 2020 USD ($) Partnership
Maximum [Member]
Long-term Purchase Commitment [Line Items]
Potential termination fee if the Merger does not occur	$ 115.0
Capital Additions [Member]
Long-term Purchase Commitment [Line Items]
Purchase contract, commitment	$ 42.2
Capital Additions [Member] | Minimum [Member]
Long-term Purchase Commitment [Line Items]
Construction contract, period (in months)	12 months
Capital Additions [Member] | Maximum [Member]
Long-term Purchase Commitment [Line Items]
Construction 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	$ 45.0	$ 31.0
Estimated Additional Costs To Be Paid
Long-term Purchase Commitment [Line Items]
Potential merger costs	46.1
Write off of Assets and Liabilities
Long-term Purchase Commitment [Line Items]
Potential merger costs	$ 1.1