Debt (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Debt Instrument [Line Items]
Debt	$ 2,421,066,000		$ 2,421,066,000		$ 2,570,296,000
Less current portion	33,936,000		33,936,000		71,676,000
Long-term debt	2,387,130,000		2,387,130,000		2,498,620,000
Maturity date			Jan. 31, 2016
Net proceeds from issuance			308,300,000
Initial conversion rate (in shares per $1,000 of principal)			34.1006
Equivalent initial conversion price (in dollars per share)	$ 29.325		$ 29.325
Conversion terms			Holders may convert their Notes at their option prior to the close of business on the second trading day immediately preceding the stated maturity date only under the following circumstances: i during any fiscal quarter commencing after the fiscal quarter ending September 30, 2011, if the last reported sale price of the Company's common stock for at least 20 trading days whether or not consecutive during a period of 30 consecutive trading days ending on the last trading day of the preceding fiscal quarter is greater than or equal to 130 of the applicable conversion price on each applicable trading day; ii during the five business day period after any five consecutive trading day period the “measurement period”, in which the trading price per 1,000 principal amount of notes for each trading day of that measurement period was less than 98 of the product of the last reported sale price of the Company's common stock and the applicable conversion rate on each such day; or iii upon the occurrence of specified corporate events. On and after March 15, 2018, until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their Notes at any time, regardless of the foregoing circumstances. Unconverted Notes mature at par in June 2018.
Imputed interest rate used in determining carrying value	7.50%		7.50%
Expected life of convertible debt	7		7
Derivative [Line Items]
Cash collateral pledged to counterparties pursuant to hedge agreements		800,000		800,000	0
Change in fair value of derivatives and amortization	(2,635,000)	(2,207,000)	(2,643,000)	(4,847,000)
Number of cap agreements maturing			5
Aggregate notional amount, matured cap agreements			315,700,000
Number of cap agreements, extended maturity			12
Aggregate notional amount, extended maturities cap agreements			83,800,000
Line of Credit Facility [Line Items]
Maturity date			Jan. 31, 2016
Line of credit borrowings outstanding	0		0
Loss on extinguishment of debt	(15,254,000)	(682,000)	(18,148,000)	(701,000)
Mortgages payable due 2012 through 2020 [Member]
Debt Instrument [Line Items]
Debt	1,175,739,000		1,175,739,000		1,342,931,000
Maturity date, start			2012
Maturity date, end			2020
Weighted average interest rate (in hundredths)	5.14%		5.14%		5.32%
Unamortized debt discount	700,000		700,000
Notes Payable, Other Payables [Member]
Debt Instrument [Line Items]
Debt	150,000,000		150,000,000		150,000,000
Maturity date			Aug. 31, 2013
Number of communities securing debt (in number of communities)			5
Amount of letter of credit securing debt			3,000,000
Description of variable rate basis			LIBOR
Basis spread on variable rate basis (in hundredths)	0.88%		0.88%
Line of Credit Facility [Line Items]
Maturity date			Aug. 31, 2013
Basis of variable rate			LIBOR
Margin on variable rate interest	0.88%		0.88%
Mortgages payable due 2012 [Member]
Debt Instrument [Line Items]
Debt	0		0		210,897,000
Maturity date			Jul. 31, 2012
Weighted average interest rate (in hundredths)	5.57%		5.57%		5.64%
Line of Credit Facility [Line Items]
Maturity date			Jul. 31, 2012
Discount Mortgage Note payable due 2013 [Member]
Debt Instrument [Line Items]
Debt	79,595,000		79,595,000		79,275,000
Maturity date			Dec. 31, 2013
Weighted average interest rate (in hundredths)	2.51%		2.51%		2.55%
Unamortized debt discount	3,900,000		3,900,000
Line of Credit Facility [Line Items]
Maturity date			Dec. 31, 2013
Variable rate tax exempt bonds [Member]
Debt Instrument [Line Items]
Debt	100,711,000		100,711,000		100,841,000
Maturity date			Dec. 31, 2032
Weighted average interest rate (in hundredths)	1.67%		1.67%		1.73%
Line of Credit Facility [Line Items]
Maturity date			Dec. 31, 2032
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Debt	362,397,000		362,397,000		371,172,000
Maturity date			Dec. 31, 2024
Weighted average interest rate (in hundredths)	8.59%		8.59%		8.60%
Line of Credit Facility [Line Items]
Maturity date			Dec. 31, 2024
Variable rate mortgage note [Member]
Debt Instrument [Line Items]
Debt	315,180,000		315,180,000		315,180,000
Maturity date			Aug. 31, 2012
Number of communities securing debt (in number of communities)			15
Description of variable rate basis			LIBOR
Basis spread on variable rate basis (in hundredths)	0.70%		0.70%
Amount of guaranty securing debt	11,500,000		11,500,000
Line of Credit Facility [Line Items]
Maturity date			Aug. 31, 2012
Basis of variable rate			LIBOR
Margin on variable rate interest	0.70%		0.70%
Convertible Debt [Member]
Debt Instrument [Line Items]
Debt	237,444,000		237,444,000		0
Maturity date			Jun. 30, 2018
Weighted average interest rate (in hundredths)	2.75%		2.75%
Principal	316,250,000		316,250,000
Unamortized discount	(78,806,000)		(78,806,000)
Net carrying amount	237,444,000		237,444,000
Equity component	78,806,000		78,806,000
Coupon interest	392,000		392,000
Amortization of discount	0		0
Interest expense related to convertible notes	392,000		392,000
Number of shares of common stock covered by hedging transactions (in shares)	10,784,315		10,784,315
Number of warrants to acquire common stock sold to Hedge Counterparties (in shares)	10,784,315		10,784,315
Strike price of warrants (in dollars per share)	$ 40.25		$ 40.25
Net cost of hedging transaction	31,900,000		31,900,000
Line of Credit Facility [Line Items]
Maturity date			Jun. 30, 2018
Interest Rate Swap [Member]
Derivative [Line Items]
Current notional balance		177,940,000		177,940,000
Highest possible notional		177,940,000		177,940,000
Lowest interest rate (in hundredths)		0.87%		0.87%
Highest interest rate (in hundredths)		5.49%		5.49%
Average fixed rate		1.60%		1.60%
Earliest maturity date		2013		2013
Latest maturity date		2016		2016
Weighted average original maturity		3.2 Y		3.2 Y
Estimated liability fair value (included in other liabilities)		(1,692,000)		(1,692,000)
Estimated asset fair value (included in other assets)		281,000		281,000
Notional amount of new agreements		28,000,000		28,000,000
Interest Rate Cap [Member]
Derivative [Line Items]
Current notional balance		693,948,000		693,948,000
Highest possible notional		693,948,000		693,948,000
Lowest interest rate (in hundredths)		5.50%		5.50%
Highest interest rate (in hundredths)		6.50%		6.50%
Average fixed rate		6.11%		6.11%
Earliest maturity date		2012		2012
Latest maturity date		2013		2013
Weighted average original maturity		3.2 Y		3.2 Y
Estimated liability fair value (included in other liabilities)		0		0
Estimated asset fair value (included in other assets)		157,000		157,000
Notional amount of new agreements		64,100,000		64,100,000
Credit Facility 2010 [Member]
Line of Credit Facility [Line Items]
Initial commitment			100,000,000
Credit Facility, maximum borrowing capacity	120,000,000		120,000,000
Description of applicable margin calculation based on utilization percentage			Amounts drawn under the facility bore interest at 90-day LIBOR plus an applicable margin, as described below. For purposes of determining the interest rate, in no event would LIBOR be less than 2.0. The applicable margin varied with the percentage of the total commitment drawn, with a 4.5 margin at 35 or lower utilization, a 5.0 margin at utilization greater than 35 but less than or equal to 50, and a 5.5 margin at greater than 50 utilization.
Quarterly commitment fee (in hundredths)			1.00%
Credit Facility 2011 [Member]
Line of Credit Facility [Line Items]
Initial commitment			200,000,000
Credit Facility, maximum borrowing capacity	230,000,000		230,000,000
Secured and unsecured letter of credit facilities
Line of Credit Facility [Line Items]
Credit Facility, maximum borrowing capacity	82,500,000		82,500,000
Letters of credit outstanding	72,100,000		72,100,000
First mortgage loan issued in March 2011 [Member]
Line of Credit Facility [Line Items]
Principal amount of note	28,000,000		28,000,000
Interest rate	5.49%		5.49%
Extinguishment of debt with proceeds of debt			28,000,000
Extinguishment of debt with release of entrance fee escrows			37,900,000
Extinguishment of debt and moved into borrowing base of credit facility			48,700,000
Extinguishment of debt with proceeds from convertible debt offering			274,900,000
Subsequent debt under master Credit Facility [Member]
Debt Instrument [Line Items]
Maturity date			Aug. 01, 2018
Description of variable rate basis			30-day LIBOR
Basis spread on variable rate basis (in hundredths)	1.82%		1.82%
Line of Credit Facility [Line Items]
Maturity date			Aug. 01, 2018
Amortization period (in years)	30		30
Principal amount of note	437,800,000		437,800,000
Interest rate	4.25%		4.25%
Extinguishment of debt with proceeds of debt			$ 445,200,000
Number of communities securing debt (in number of communities)			44
Percentage of loans bearing interest at a fixed rate (in hundredths)	75.00%		75.00%
Percentage of loans bearing interest at a variable rate (in hundredths)	25.00%		25.00%
Basis of variable rate			30-day LIBOR
Margin on variable rate interest	1.82%		1.82%