Debt (Details) (USD $)	3 Months Ended			3 Months Ended	12 Months Ended		3 Months Ended								3 Months Ended		3 Months Ended		3 Months Ended		3 Months Ended		3 Months Ended		3 Months Ended		3 Months Ended		3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Mar. 31, 2012 Credit Facility 2011 [Member]	Dec. 31, 2011 Credit Facility 2011 [Member]	Mar. 31, 2012 Secured and unsecured letter of credit facilities	Mar. 31, 2012 Other Financing Transaction [Member]	Mar. 31, 2012 First mortgage loan issued in February 2012 [Member]	Mar. 31, 2012 4.2% First Mortgage Facility [Member] Integer	Mar. 31, 2012 7.0% First Mortgage Facility [Member] Integer	Mar. 31, 2012 Interest Rate Swap [Member] Integer	Dec. 31, 2011 Interest Rate Swap [Member]	Mar. 31, 2012 Interest Rate Cap [Member] Integer	Dec. 31, 2011 Interest Rate Cap [Member]	Mar. 31, 2012 Mortgages payable due [Member]	Dec. 31, 2011 Mortgages payable due [Member]	Mar. 31, 2012 Notes Payable, Other Payables [Member] Integer	Dec. 31, 2011 Notes Payable, Other Payables [Member]	Mar. 31, 2012 Discount Mortgage Note payable due 2013 [Member]	Dec. 31, 2011 Discount Mortgage Note payable due 2013 [Member]	Mar. 31, 2012 Variable rate tax exempt bonds [Member]	Dec. 31, 2011 Variable rate tax exempt bonds [Member]	Mar. 31, 2012 Capital Lease Obligations [Member]	Dec. 31, 2011 Capital Lease Obligations [Member]	Mar. 31, 2012 Convertible Debt [Member]	Dec. 31, 2011 Convertible Debt [Member]	Mar. 31, 2012 Construction Financing [Member]	Dec. 31, 2011 Construction Financing [Member]	Mar. 31, 2012 Notes Payable, Insurance Premiums Due 2012 [Member]	Dec. 31, 2011 Notes Payable, Insurance Premiums Due 2012 [Member]
Debt Instrument [Line Items]
Debt	$ 2,506,242,000		$ 2,398,625,000												$ 1,573,629,000	$ 1,470,462,000	$ 147,554,000	$ 148,601,000	$ 80,068,000	$ 79,911,000	$ 100,288,000	$ 100,423,000	$ 342,643,000	$ 348,195,000	$ 244,186,000	$ 241,897,000	$ 7,194,000	$ 6,591,000	$ 10,680,000	$ 2,545,000
Less current portion	125,316,000		47,654,000
Long-term debt	2,380,926,000		2,350,971,000
Maturity date				Jan. 31, 2016			Jan. 31, 2017	Feb. 28, 2017									Aug. 31, 2013		Dec. 31, 2013		Dec. 31, 2032		Dec. 31, 2026		Jun. 30, 2018		Dec. 31, 2024		Dec. 31, 2012
Maturity date, start															2013
Maturity date, end															2022
Weighted average interest rate (in hundredths)															4.90%	5.04%			2.57%	2.52%	1.80%	1.65%	8.62%	8.61%	2.75%	2.75%	7.00%	7.00%	2.64%	3.11%
Unamortized debt discount															500,000				2,500,000						72,100,000
Number of communities securing debt (in number of communities)																	5
Amount of cash collateral securing debt																	3,000,000
Description of variable rate basis																	LIBOR plus0.88%
Basis spread on variable rate basis (in hundredths)																	0.88%
Principal																									316,300,000
Derivative [Line Items]
Current notional balance											27,660,000		418,238,000
Highest possible notional											27,660,000		418,238,000
Lowest interest rate (in hundredths)											5.49%		5.00%
Highest interest rate (in hundredths)											5.49%		6.06%
Average fixed rate											5.49%		5.49%
Earliest maturity date											2016		2012
Latest maturity date											2016		2017
Weighted average original maturity											5.0 years		2.7 years
Estimated liability fair value (included in other liabilities)											(1,793,000)	(2,809,000)
Estimated asset fair value (included in other assets)													0	0
Change in fair value of derivatives and amortization	(233,000)	(8,000)
Notional amount of new agreements													169,500,000
Number of swaps agreements terminated											1
Notional amount of swap agreement terminated											150,000,000
Number of cap agreements entered													2
Cash collateral paid to counterparties pursuant to swap termination											1,200,000
Line of Credit Facility [Line Items]
Commitment on Prior borrowing facility					120,000,000
Initial commitment				200,000,000
Credit Facility, maximum borrowing capacity				230,000,000		82,700,000
Maturity date				Jan. 31, 2016			Jan. 31, 2017	Feb. 28, 2017									Aug. 31, 2013		Dec. 31, 2013		Dec. 31, 2032		Dec. 31, 2026		Jun. 30, 2018		Dec. 31, 2024		Dec. 31, 2012
Description of applicable margin calculation based on utilization percentage				Amounts drawn under the facility bear interest at 90-day LIBOR plus an applicable margin, as described below. For purposes of determining the interest rate, in no event will LIBOR be less than 2.0%. The applicable margin varies 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. The Company is also required to pay a quarterly commitment fee of 1.0% per annum on the unused portion of the facility.
Availability under the secured line of credit				199,800,000
Line of credit borrowings outstanding				85,000,000
Letters of credit outstanding						77,800,000
Principal amount of note							63,000,000	20,000,000	77,900,000	15,000,000
Interest rate terms							The loan bears interest at a variable rate equal to 30-day LIBOR plus a margin of 300 basis points and matures in January 2017.	The loan bears interest at a variable rate equal to 30-day LIBOR plus a margin of 275 basis points and matures in February 2017.	75% of the facility bears interest at a fixed rate of 4.2% and the remaining 25% of the facility bears interest at a variable rate of 30-day LIBOR plus a margin of 276 basis points	7.0%
Extinguishment of debt with proceeds of debt							$ 62,800,000
Percentage of loans bearing interest at a fixed rate (in hundredths)									75.00%
Percentage of loans bearing interest at a variable rate (in hundredths)									25.00%
Basis of variable rate																	LIBOR plus0.88%
Margin on variable rate interest																	0.88%
Term of loan									10	2