Long-term debt and cash flow hedges (Details) (USD $)	3 Months Ended	9 Months Ended	12 Months Ended										9 Months Ended		9 Months Ended		9 Months Ended							9 Months Ended		9 Months Ended			1 Months Ended				1 Months Ended	9 Months Ended						9 Months Ended							9 Months Ended	5 Months Ended
	Mar. 31, 2011 Beds	Sep. 30, 2012	Dec. 31, 2010 Beds	Dec. 31, 2007	Sep. 21, 2012	Dec. 31, 2011	Sep. 29, 2010	Mar. 31, 2011 Interest Rate Cap from December Twenty Eleven to December Twenty Twelve [Member]	Mar. 31, 2011 Interest Rate Cap from December Twenty Twelve to December Twenty Thirteen [Member]	Sep. 30, 2012 Interest Rate Swap [Member]	Mar. 31, 2011 Interest Rate Swap [Member]	Mar. 31, 2011 Interest Rate Swap Effective in March Twenty Eleven [Member]	Sep. 30, 2012 Interest Rate Swap One Effective in December Twenty Eleven [Member]	Mar. 31, 2011 Interest Rate Swap One Effective in December Twenty Eleven [Member]	Sep. 30, 2012 Interest Rate Swap Two Effective in December Twenty Eleven [Member]	Mar. 31, 2011 Interest Rate Swap Two Effective in December Twenty Eleven [Member]	Sep. 30, 2012 Interest Rate Swap Three Effective in December Twenty Eleven [Member]	Mar. 31, 2011 Interest Rate Swap Three Effective in December Twenty Eleven [Member]	Mar. 15, 2011 Federal Funds Rate [Member]	Mar. 15, 2011 Euro Dollar Rate [Member]	Sep. 29, 2010 Debtor Optional Redemption Period 1 [Member]	Sep. 29, 2010 Debtor Optional Redemption Period 2 [Member]	Sep. 29, 2010 Debtor Optional Redemption Period 3 [Member]	Sep. 30, 2012 Senior Unsecured Notes Seven Point Zero Zero Percent [Member]	Sep. 29, 2010 Senior Unsecured Notes Seven Point Zero Zero Percent [Member]	Sep. 30, 2012 Senior Notes Seven Point One Two Five Percent [Member]	Jun. 30, 2008 Senior Notes Seven Point One Two Five Percent [Member]	Jun. 30, 2006 Senior Notes Seven Point One Two Five Percent [Member]	Nov. 15, 2010 Revolving Credit Facility [Member]	Sep. 30, 2012 Revolving Credit Facility [Member]	Sep. 21, 2012 Revolving Credit Facility [Member]	Nov. 15, 2010 Revolving Credit Facility [Member] Letter of Credit [Member]	Nov. 15, 2010 Term Loan A [Member]	Sep. 30, 2012 Term Loan A [Member]	Sep. 21, 2012 Term Loan A [Member]	Sep. 30, 2012 Term Loan A [Member] Euro Dollar Rate [Member]	Sep. 30, 2012 Term Loan A [Member] ABR-based loans [Member]	Sep. 30, 2012 Term Loan A [Member] Minimum [Member]	Sep. 30, 2012 Term Loan A [Member] Maximum [Member]	Sep. 30, 2012 Term Loan A2 [Member]	Sep. 21, 2012 Term Loan A2 [Member]	Sep. 21, 2012 Term Loan B [Member]	Sep. 30, 2012 Term Loan B [Member] Euro Dollar Rate [Member]	Sep. 30, 2012 Term Loan B [Member] Minimum [Member]	Sep. 30, 2012 Term Loan B [Member] Maximum [Member]	Sep. 30, 2012 Term Loan B [Member] Maximum [Member] Euro Dollar Rate [Member]	Sep. 30, 2012 Accounts Receivable Securitization Facility [Member]	May 31, 2012 Accounts Receivable Securitization Facility [Member] Commercial Paper Rate [Member]	Oct. 31, 2010 Accounts Receivable Securitization Facility [Member] Commercial Paper Rate [Member]	May 31, 2012 Accounts Receivable Securitization Facility [Member] Minimum [Member]	Oct. 31, 2010 Accounts Receivable Securitization Facility [Member] Minimum [Member]	May 31, 2012 Accounts Receivable Securitization Facility [Member] Maximum [Member]	Oct. 31, 2010 Accounts Receivable Securitization Facility [Member] Maximum [Member]
Long Term Debt And Cash Flow Hedges (Textual) [Abstract]
Senior secured facility principal payments																																		$ 943,000,000
Line of credit facility, maturity date																													Nov. 15, 2015				Nov. 15, 2015														Oct. 25, 2013
Interest rate																			0.50%	1.00%																				100.00%									0.475%
Consolidated leverage ratio																																						0.50%	1.25%					1.75%	2.00%
Applicable margin rate																																				1.75%	0.75%	1.50%	2.25%				2.75%	2.75%	3.00%	1.00%
Fee paid on amount outstanding																																																0.375%
Available borrowing capacity		678,000,000
Line of credit facility amount outstanding		15,000,000																												63,000,000																	60,000,000
Letter of credit amount outstanding		59,000,000
Commitment percentage for facility fee																																																102.00%
Line of credit facility additional borrowing capacity																											150,000,000																				215,000,000				200,000,000		240,000,000
Senior secured facility borrowing capacity increased																																																		35,000,000		275,000,000
Senior notes, issued																									250,000,000			250,000,000
Senior notes, interest rate																									7.00%			7.125%
Senior notes, maturity date																								Oct. 01, 2018		Jun. 30, 2016
Unsecured Notes make-whole redemption date																					Oct. 01, 2014	Oct. 01, 2015	Oct. 01, 2016
Unsecured Notes make-whole redemption price																					103.50%	101.75%	100.00%
Ownership interest percentage							100.00%
Carrying amount of debt		3,440,962,000				3,651,428,000
Fair value of debt		3,530,000,000
Notional amount of interest rate cash flow hedges			600,000,000	75,000,000				450,000,000	400,000,000		425,000,000	225,000,000		100,000,000		25,000,000		75,000,000
Derivative effective date	2011-03		2011-12
Premium paid on interest rate cap	740,000
Derivative cap interest rate	7.00%
Number of interest rate swap	6		4
Interest rate maturity		2015-05	2015-05	2012-10									2014-12		2013-12		2012-12
Average fixed interest rate	1.91%		2.38%
Fixed interest rate				4.76%										2.50%		1.96%		1.32%
Fair value of interest rate swap										46,000,000
Proceeds from the New Term Loan																																								700,000,000
Variable Rate Basis		1% LIBOR floor
Deferred financing costs		29,000,000
Maximum Borrowing Capacity					3,430,000,000																								800,000,000		777,000,000	125,000,000			988,000,000						900,000,000	746,000,000
Commitment Fee Amount		23,000,000
Current Borrowing Capacity					$ 45,000,000