Organization and Summary of Significant Accounting Policies (Textual) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Dec. 31, 2018 shares	Dec. 31, 2018	Dec. 31, 2018 Property	Dec. 31, 2018 States	Dec. 31, 2018 Communities	Dec. 31, 2018 ft²
Real Estate Properties
Percentage Of Rental Revenue And Other Income From Leasing Of Apartment Homes	93.00%
Percentage of Rental Revenue and Other Income From Non-leasing Activities	7.00%
Loss Contingency Accrual	$ 8,700	$ 32,100
General partners' capital account, units outstanding | shares		113,643,166		113,844,267
Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest	96.50%	96.40%
Number of Real Estate Properties						303		303
Advertising expenses	$ 20,200	$ 18,800	$ 13,000
Number of States in which Entity Operates | States							17
Minimum
Real Estate Properties
Percentage of ownership interests of all consolidated subsidiaries (percent)					92.50%
Maximum
Real Estate Properties
Percentage of ownership interests of all consolidated subsidiaries (percent)					100.00%
Development Properties
Real Estate Properties
Number of Real Estate Properties | Communities								3
Number of units under development community | Property						577
Development Properties | Expected Costs
Real Estate Properties
Development and capital improvements in progress	118,500
Development Properties | Costs Incurred to Date
Real Estate Properties
Development and capital improvements in progress	30,900
Retail
Real Estate Properties
Number of Real Estate Properties | Property						30
Square footage of real estate property (square foot) | ft²									615,000
Commercial Properties
Real Estate Properties
Number of Real Estate Properties | Property						4
Square footage of real estate property (square foot) | ft²									260,000
Post Massachusetts Avenue
Real Estate Properties
Equity Method Investment, Ownership Percentage					35.00%
Real Estate Investments, Joint Ventures	44,200
Real Estate Technology Ventures, L.P. [Member]
Real Estate Properties
Equity Method Investment, Ownership Percentage					20.70%
Real Estate Investments, Joint Ventures	3,800
Real Estate Technology Ventures, L.P. [Member] | Limited Partner Capital Contribution, Future Obligation [Member]
Real Estate Properties
Real Estate Investments, Joint Ventures	13,600
Post River North [Member]
Real Estate Properties
Number of units in real estate property					359
Buildings, Improvements, and Other	70,500
Percentage of ownership interests of all consolidated subsidiaries (percent)					92.50%
Land	14,900
Accrued Liabilities	1,200
Parent Company [Member]
Real Estate Properties
Net Cash Provided by (Used in) Operating Activities	734,292	660,800	485,004
Capitalized Development Costs, Period	4,200	11,000	2,700
Development and capital improvements in progress	59,506	116,833
Buildings, Improvements, and Other	11,670,216	11,281,504
Total interest capitalized during period	2,047	7,238	2,073
Lease Intangibles, Gross	13,598,550	13,234,754
Depreciation and amortization	489,759	493,708	322,958
Land	1,868,828	1,836,417
Accrued Liabilities	413,850	405,560
Net Cash Provided by (Used in) Investing Activities	(366,350)	(294,210)	(649,131)
Net Cash Provided by (Used in) Financing Activities	(405,136)	(399,523)	222,286
Accrued Liabilities [Member]
Real Estate Properties
Dividends Payable	113,200	108,700
Accrued Real Estate Taxes	123,500	99,600
Unearned Income	41,100	40,800
Loss Contingency, Loss in Period		32,100
Security Deposit Liability	18,700	19,100
Interest Payable	15,100	18,100
Limited Partner [Member]
Real Estate Properties
Net Cash Provided by (Used in) Operating Activities	734,292	$ 660,800	485,004
General partners' capital account, units outstanding | shares		113,643,166		113,844,267
Development and capital improvements in progress	59,506	$ 116,833
Buildings, Improvements, and Other	11,670,216	11,281,504
Total interest capitalized during period	2,047	7,238	2,073
Lease Intangibles, Gross	13,598,550	13,234,754
Depreciation and amortization	489,759	493,708	322,958
Land	1,868,828	1,836,417
Accrued Liabilities	413,850	405,560
Net Cash Provided by (Used in) Investing Activities	(366,350)	(294,210)	(649,131)
Net Cash Provided by (Used in) Financing Activities	(405,136)	(399,523)	222,286
Water Utility Revenue [Member]
Real Estate Properties
Property Management Fee Revenue	39,100	38,300	31,100
Cable Utility Revenue [Member]
Real Estate Properties
Property Management Fee Revenue	29,800	30,400	31,600
Accounting Standards Update 2016-15 [Member]
Real Estate Properties
Net Cash Provided by (Used in) Operating Activities		1,900	600
Net Cash Provided by (Used in) Investing Activities		200	500
Net Cash Provided by (Used in) Financing Activities		1,700	100
Accounting Standards Update 2016-18 [Member]
Real Estate Properties
Net Cash Provided by (Used in) Operating Activities		400	300
Net Cash Provided by (Used in) Investing Activities		10,600	61,900
Restricted Cash and Cash Equivalents		78,100	$ 88,300
Accounting Standards Update 2016-02 [Member]
Real Estate Properties
Operating Lease, Right-of-Use Asset	43,000
Operating Lease, Liability	33,000
Leases, Acquired-in-Place [Member]
Real Estate Properties
Lease Intangibles, Gross	$ 3,900	$ 7,100