Significant Transactions During the First Quarter of 2016 and Subsequent Events (Details)	1 Months Ended				3 Months Ended		12 Months Ended
	Apr. 30, 2016 USD ($) unit $ / shares shares	Mar. 31, 2016 USD ($) unit	Feb. 29, 2016 USD ($) unit	Jan. 31, 2016 USD ($) unit	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Equity Method Investments [Line Items]
Acquisitions of real estate and acquisition related capital expenditures					$ 110,309,000	$ 199,190,000
Land and land improvements		$ 2,573,923,000			2,573,923,000		$ 2,522,842,000
Buildings and improvements		$ 10,052,754,000			10,052,754,000		$ 9,808,627,000
Tax on gain on sale of real estate					4,279,000	0
Payments to acquire preferred equity investments					$ 50,591,000	$ 48,650,000
Series H Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred stock, stated interest percentage					7.125%		7.125%
Subsequent Event [Member] | Series H Preferred Stock [Member]
Schedule of Equity Method Investments [Line Items]
Preferred stock, shares outstanding | shares	2,950,000
Preferred stock, shares issued | shares	2,950,000
Preferred stock, stated interest percentage	7.125%
Cumulative redeemable preferred stock redemption price per share | $ / shares	$ 25.00
Unsecured Bonds 4.36% [Member] | Unsecured Debt [Member]
Schedule of Equity Method Investments [Line Items]
Debt instrument, face amount				$ 150,000,000
Debt Instrument, interest rate, stated percent				4.36%
Unsecured Bonds 3.375% [Member] | Unsecured Debt [Member] | Subsequent Event [Member]
Schedule of Equity Method Investments [Line Items]
Debt instrument, face amount	$ 450,000,000
Debt Instrument, interest rate, stated percent	3.375%
BEXAWE, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Preferred equity investment, ownership percentage		50.00%			50.00%
Palo Alto, CA [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds from sale of buildings				$ 18,000,000
Gain on sale of properties				$ 9,600,000
Acquisitions Q1 2016 [Member]
Schedule of Equity Method Investments [Line Items]
Acquisitions of real estate and acquisition related capital expenditures					$ 148,700,000
Land and land improvements		$ 35,600,000			35,600,000
Buildings and improvements		$ 112,200,000			112,200,000
Acquisitions Q1 2016 [Member] | Leases, Acquired-in-Place [Member]
Schedule of Equity Method Investments [Line Items]
Acquired in-place leases					900,000
Mio Apartment Community [Member]
Schedule of Equity Method Investments [Line Items]
Number of units acquired | unit				103
Acquisitions of real estate and acquisition related capital expenditures				$ 51,300,000
Form 15 [Member]
Schedule of Equity Method Investments [Line Items]
Number of units acquired | unit		242
Acquisitions of real estate and acquisition related capital expenditures		$ 97,400,000
Mortgage loans assumed in acquisition		44,800,000			44,800,000
The Heights [Member] | BEXAWE, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Gain on sale of properties				$ 7,400,000
Number of units sold | unit				332
Sales of real estate				$ 93,800,000
Repayments of secured debt				$ 50,300,000
Preferred equity investment, ownership percentage				50.00%
Harvest Park [Member]
Schedule of Equity Method Investments [Line Items]
Gain on sale of properties			$ 6,400,000
Tax on gain on sale of real estate			$ 4,300,000
Number of units sold | unit			104
Sales of real estate			$ 30,500,000
201 Lexington [Member]
Schedule of Equity Method Investments [Line Items]
Payments to acquire preferred equity investments		$ 31,800,000
Preferred stock, stated interest percentage		12.00%
201 Lexington [Member] | Commitments To Invest [Member]
Schedule of Equity Method Investments [Line Items]
Commitment to fund preferred equity investment		$ 47,100,000			$ 47,100,000
Canyon Creek [Member] | BEXAWE, LLC [Member] | Subsequent Event [Member]
Schedule of Equity Method Investments [Line Items]
Number of units sold | unit	200
Sales of real estate	$ 53,500,000
Repayments of secured debt	$ 26,300,000