Significant Transactions During the Second Quarter of 2012 and Subsequent Events (Details) (USD $)	1 Months Ended	3 Months Ended	6 Months Ended											3 Months Ended	6 Months Ended			6 Months Ended				3 Months Ended							6 Months Ended
	Jul. 31, 2012	Jun. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Jul. 31, 2012 Swap [Member]	Jun. 30, 2012 Swap [Member]	Jun. 30, 2012 Not Designated as Hedging Instrument [Member] Swap [Member]	Apr. 30, 2012 Not Designated as Hedging Instrument [Member] Swap [Member]	Dec. 31, 2011 Not Designated as Hedging Instrument [Member] Swap [Member]	Jun. 30, 2012 Before Amendment [Member]	Jun. 30, 2011 Before Amendment [Member]	Jun. 30, 2012 After Amendment [Member]	Jun. 30, 2011 After Amendment [Member]	Jun. 30, 2012 Accordion Feature [Member]	Jun. 30, 2012 Unsecured Debt [Member]	Jun. 30, 2012 Unsecured Debt [Member] Before Amendment [Member]	Jun. 30, 2012 Unsecured Debt [Member] After Amendment [Member]	Jun. 30, 2012 Mortgages [Member]	Jun. 30, 2011 Mortgages [Member]	Jun. 30, 2012 Park Catalina [Member]	Jun. 30, 2012 Huntington [Member]	Jun. 30, 2012 Essex Skyline at MacArthrur Place [Member]	Jun. 30, 2012 Montebello [Member]	Jun. 30, 2012 Montebello [Member] Mortgages [Member]	Jun. 30, 2012 Emeryville California [Member]	Jun. 30, 2012 Valley Village California [Member]	Jun. 30, 2012 San Francisco California [Member] sqft	Jun. 30, 2012 San Mateo California [Member]	Jun. 30, 2012 Cupertino California [Member]
Acquisitions [Abstract]
Number of units																				90	276	349	248		190	121	463	197
Cost of acquired entity																				$ 23,700,000	$ 48,300,000	$ 85,000,000	$ 52,000,000		$ 58,200,000	$ 37,600,000	$ 250,000,000	$ 76,100,000
Interest and other income on business acquisition																						2,300,000
Gain on remeasurement co-investment interest																						21,900,000
Early repayment of secured loan																						80,000,000
Secured Debt		1,615,645,000	1,615,645,000	1,745,858,000																	30,300,000			26,500,000
Fixed Rate																					5.70%		5.60%
Loan premium debt at fair value																					4,300,000		4,100,000
Area of real estate property (in square feet)																											9,300
Noncontrolling interest ownership percentage																											55.00%	55.00%
Effective interest rate percentage (in hundredths)																					3.30%		3.10%
Deposit to purchase property																										1,000,000
Preferred equity investment in related party																													14,000,000
Preferred return rate (in hundredths)			9.50%																										9.50%
Preferred equity investment maturity term		May 2016	May 2016
Additional preferred equity investment to fund renovation costs																													4,000,000
Common stock [Abstract]
Number of common shares sold (in shares)	1,769,989	920,281
Proceeds from sale of common shares	268,000,000	139,400,000	170,944,000
Average price per share of common shares sold (dollars per share)	$ 153.01	$ 152.94
Debt Disclosure [Abstract]
Repayments of loans																		137,700,000				80,000,000
Weighted average interest rate (in hundredths)																		5.60%
Losses from early retirement of debt																		1,500,000
Debt maturity																			Jun. 30, 2013
Prepayment penalties and write-offs of deferred charges																		1,000,000
Unsecured Term Loan											200,000,000		350,000,000
Debt Instrument, Basis Spread on Variable Rate										1.425%		1.30%		1.20%
Debt Instrument, Description of Variable Rate Basis			LIBOR											LIBOR
Debt instrument increase in capacity		150,000,000	150,000,000
Interest rate cap contracts notional amount															100,000,000
Interest rate (in hundredths)					2.20%	2.20%	2.46%	2.46%	2.66%						2.20%
Unsecured Line of Credit														600,000,000		425,000,000	500,000,000
Unsecured Line of Credit expiration date															Dec. 31, 2015
Unsecured line of credit facility fee percentage		2000.00%	2000.00%
Derivative [Line Items]
Notional amount fixed spread basis							125,000,000		150,000,000
Amount of hedged item							$ 200,000,000		$ 200,000,000
Interest rate (in hundredths)					2.20%	2.20%	2.46%	2.46%	2.66%						2.20%