Real Estate (Details) (USD $)	3 Months Ended		9 Months Ended		45 Months Ended		3 Months Ended		9 Months Ended									3 Months Ended		9 Months Ended				3 Months Ended		9 Months Ended	45 Months Ended		9 Months Ended					3 Months Ended	9 Months Ended	3 Months Ended	12 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2013	Mar. 31, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Retail Site [Member]	Sep. 30, 2013 Retail Site [Member]	Sep. 30, 2014 Retail Site [Member]	Sep. 30, 2013 Retail Site [Member]	Sep. 30, 2014 Assets [Member]	Dec. 31, 2013 Assets [Member]	Mar. 31, 2013 Assets [Member]	Sep. 30, 2014 Liability [Member]	Dec. 31, 2013 Liability [Member]	Mar. 31, 2013 Liability [Member]	Mar. 31, 2013 Consolidated Properties [Member]	Sep. 30, 2014 Ground Lease [Member]	Sep. 30, 2013 Ground Lease [Member]	Sep. 30, 2014 Ground Lease [Member]	Sep. 30, 2013 Ground Lease [Member]	Mar. 31, 2013 Building and Building Improvements [Member]	Mar. 31, 2013 Furniture and Fixtures [Member]	Sep. 30, 2013 Archstone Transaction [Member]	Mar. 31, 2013 Archstone Transaction [Member]	Sep. 30, 2013 Archstone Transaction [Member]	Sep. 30, 2014 Archstone Transaction [Member]	Mar. 31, 2013 Unconsolidated Properties [Member]	Sep. 30, 2013 Investment Banking and Legal/Accounting Fees [Member]	Sep. 30, 2014 Projects under development [Member]	Dec. 31, 2013 Projects under development [Member]	Sep. 30, 2014 Land held for development [Member]	Dec. 31, 2013 Land held for development [Member]	Sep. 30, 2013 Limited Partner [Member]	Sep. 30, 2013 Limited Partner [Member]	Mar. 31, 2013 Archstone Transaction [Member]	Dec. 31, 2012 Public Offering [Member]	Sep. 30, 2014 Term Loan [Member]	Sep. 30, 2014 Consolidated Rental Properties [Member]	Dec. 31, 2013 Consolidated Rental Properties [Member]
Land			$ 6,329,907,000		$ 6,329,907,000	$ 6,192,512,000
Property, Plant and Equipment, Gross [Abstract]
Investment Building and Building Improvements			18,064,739,000		18,064,739,000	17,509,609,000
Fixtures and Equipment, Gross			1,345,371,000		1,345,371,000	1,214,220,000
Finite-Lived Intangible Asset, Acquired-in-Place Leases			509,499,000		509,499,000	502,218,000
Land Under Development																														395,402,000	353,574,000
Projects under development			1,046,210,000		1,046,210,000	988,867,000																								650,808,000	635,293,000	50,878,000	52,133,000
Land held for development			279,139,000		279,139,000	393,522,000																										228,261,000	341,389,000
Real Estate Investment Property, at Cost			27,574,865,000		27,574,865,000	26,800,948,000
Real Estate Investment Property, Accumulated Depreciation			(5,314,260,000)		(5,314,260,000)	(4,807,709,000)
Investment in real estate, net			22,260,605,000		22,260,605,000	21,993,239,000
Below Market Lease, Gross											178,251,000	178,251,000	178,251,000	5,270,000	5,500,000	8,040,000
Finite-Lived Intangible Asset, Off-market Lease, Favorable, Gross		1,260,000	1,260,000		1,260,000	1,260,000
Finite-Lived Intangible Assets, Gross			179,511,000		179,511,000	179,511,000
Amortization of Intangible Assets			(7,777,000)		(7,777,000)	(4,364,000)
Lease Intangible Assets, Net			171,734,000		171,734,000	175,147,000
Off-market Lease, Unfavorable														2,400,000	2,400,000	2,400,000
Lease Intangible Liabilities			7,670,000		7,670,000	7,900,000
Amortization Of Intangible Liabilities			(1,968,000)		(1,968,000)	(1,161,000)
Lease Intangible Liabilities, net			5,702,000		5,702,000	6,739,000
Amortization of Intangible Assets			2,376,000	2,214,000			300,000	100,000	900,000	300,000								1,000,000	1,100,000	3,200,000	2,500,000
Weighted Average Ground Lease Amortization Period			49.8		49.8
Weighted Average Retail Lease Amortization Period			2.8		2.8
Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year			846,000		846,000		(234,000)		(234,000)									1,080,000		1,080,000
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months			3,382,000		3,382,000		(939,000)		(939,000)									4,321,000		4,321,000
Finite-Lived Intangible Assets, Amortization Expense, Year Two			3,425,000		3,425,000		(896,000)		(896,000)									4,321,000		4,321,000
Finite-Lived Intangible Assets, Amortization Expense, Year Three			3,781,000		3,781,000		(540,000)		(540,000)									4,321,000		4,321,000
Finite-Lived Intangible Assets, Amortization Expense, Year Four			4,250,000		4,250,000		(71,000)		(71,000)									4,321,000		4,321,000
Finite-Lived Intangible Assets, Amortization Expense, Year Five			4,250,000		4,250,000		(71,000)		(71,000)									4,321,000		4,321,000
Business Acquisition, Percentage of Voting Interests Acquired																									60.00%
Properties acquired			4																						71														4
Property Units Acquired			1,080																						20,160														1,080
Partially Owned Properties Acquired Properties																									1
Partially Owned Units Acquired Units																									432
Partially Owned Unconsolidated Properties Acquired																									1
Partially Owned Unconsolidated Units Acquired																									336
Master Leased Properties Acquired																									3
Master Leased Units Acquired																									853
Uncompleted Development Properties Acquired																	2								4			2
Uncompleted Development Units Acquired																									964
Land parcels acquired			2																						14
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net																									8,968,389,000
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual																								153,500,000		358,000,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual																								104,200,000		244,200,000
Payments to Acquire Businesses, Net of Cash Acquired			0	4,000,875,000																					4,000,000,000
Mortgage notes payable			5,090,960,000		5,090,960,000	5,174,166,000																			2,000,000,000
Issuance of Common Shares																																				34,468,085	21,850,000
Stock Issued During Period, Value, New Issues																																				1,900,000,000	1,200,000,000
Average Price Per Common Share Issued																																				$ 55.99	$ 54.75
Noncash or Part Noncash Acquisition, Debt Assumed																									3,100,000,000
Mark-to-Market Debt Premiums		127,900,000
Proceeds From Sale Of Property			227,602,000																																				197,100,000	4,500,000,000
Term Loan Principal Amount																																						750,000,000
Number of Properties Subject to Ground Leases		9
Number of Real Estate Properties			396		396
Business Acquisition, Purchase Price Allocation, Land																									2,239,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment																						5,765,538,000	61,470,000
Business Acquisition, Purchase Price Allocation, Amortizable Intangible Assets																									304,830,000
Business Combination Purchase Price Allocation Projects Under Development																									36,583,000
Business Combination Purchase Price Allocation Land Held for Development																									244,097,000
Business Combination Purchase Price Allocation Investments in Unconsolidated Entities																									230,608,000
Business Acquisition, Purchase Price Allocation, Other Assets																									195,260,000
Business Acquisition, Purchase Price Allocation, Other Liabilities																									(108,997,000)
Business Combination, Acquisition Related Costs			300,000	200,000																							101,100,000
Payments of Financing Costs			10,598,000	18,254,000																						2,500,000	13,500,000
Payments for Merger Related Costs				19,700,000	87,600,000																								19,700,000
Income Loss From Equity Method Investments Merger Expenses			6,400,000	54,800,000
Business Acquisition, Pro Forma Revenue	629,446,000			1,854,509,000																														629,446,000	1,854,509,000
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax	84,978,000			132,592,000																														84,978,000	132,592,000
Business Acquisition, Proforma Income Loss From Discontinued Operations Net Of Tax	403,181,000			2,024,346,000																														403,181,000	2,024,346,000
Business Acquisition, Pro Forma Net Income (Loss)	488,159,000			2,156,938,000																														488,159,000	2,156,938,000
Business Acquisition, Proforma Net Income Available to Common Shares	468,959,000			2,070,153,000
Business Acquisition, Proforma Net Income Available to Units																																		487,433,000	2,155,736,000
Business Acquisition, Pro Forma Earnings Per Share, Basic	$ 1.30			$ 5.76																														$ 1.30	$ 5.76
Business Acquisition, Pro Forma Earnings Per Share, Diluted	$ 1.30			$ 5.74																														$ 1.30	$ 5.74
Business Acquisition, Proforma Weighted Average Common Shares Basic	359,811,000			359,611,000
Business Acquisition, Proforma Weighted Average Number Of Shares Diluted	375,883,000			375,808,000
Business Acquisition, Proforma Weighted Average Limited Partnership And General Partnership Units Outstanding Basic																																		373,547,000	373,347,000
Business Acquisition, Proforma Weighted Average Number Of Limited Partnership And General Partnership Unit Outstanding Diluted																																		375,883,000	375,808,000
Severance Costs																										$ 53,600,000
land parcel disposed			2