Long-term debt and cash flow hedges - Additional Information (Detail) (USD $)							3 Months Ended				3 Months Ended							3 Months Ended				1 Months Ended										1 Months Ended				1 Months Ended							1 Months Ended	6 Months Ended						1 Months Ended		3 Months Ended	6 Months Ended								1 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Mar. 31, 2011	Nov. 15, 2010	Mar. 31, 2011 Interest Rate Cap from December Twenty Eleven to December Twenty Twelve	Mar. 31, 2011 Interest Rate Cap from December Twenty Twelve to December Twenty Thirteen	Mar. 31, 2011 Interest Rate Swap Derivative	Dec. 31, 2010 Interest Rate Swap Derivative	Dec. 31, 2007 Interest Rate Swap	Jun. 30, 2012 Interest Rate Swap	Mar. 31, 2011 Interest Rate Swap Effective in March Twenty Eleven Derivative	Mar. 31, 2011 Interest Rate Swap One Effective in December Twenty Eleven	Mar. 31, 2011 Interest Rate Swap Two Effective in December Twenty Eleven	Mar. 31, 2011 Interest Rate Swap Three Effective in December Twenty Eleven	Mar. 15, 2011 Federal Funds Rate	Mar. 15, 2011 One Month Eurodollar Rate	Jun. 30, 2012 ABR-based loans	Mar. 31, 2011 Minimum Interest Rate Cap from December Twenty Eleven to December Twenty Twelve	Mar. 31, 2011 Minimum Interest Rate Cap from December Twenty Twelve to December Twenty Thirteen	Mar. 31, 2011 Maximum Interest Rate Cap from December Twenty Eleven to December Twenty Twelve	Mar. 31, 2011 Maximum Interest Rate Cap from December Twenty Twelve to December Twenty Thirteen	Sep. 29, 2010 Senior Unsecured Notes Seven Point Zero Zero Percent	Sep. 29, 2010 Senior Unsecured Notes Seven Point Zero Zero Percent Semi Annual Payment, First Payment	Sep. 29, 2010 Senior Unsecured Notes Seven Point Zero Zero Percent Semi Annual Payment, Second Payment	Sep. 29, 2010 Senior Unsecured Notes Seven Point Zero Zero Percent Treasury Rate	Sep. 29, 2010 Senior Unsecured Notes Seven Point Zero Zero Percent Debtor Optional Redemption Period 1	Sep. 29, 2010 Senior Unsecured Notes Seven Point Zero Zero Percent Debtor Optional Redemption Period 2	Sep. 29, 2010 Senior Unsecured Notes Seven Point Zero Zero Percent Debtor Optional Redemption Period 3	Sep. 29, 2010 Senior Unsecured Notes Seven Point Zero Zero Percent Minimum Debtor Optional Redemption Period 1	Sep. 29, 2010 Senior Unsecured Notes Seven Point Zero Zero Percent Minimum Debtor Optional Redemption Period 2	Sep. 29, 2010 Senior Unsecured Notes Seven Point Zero Zero Percent Minimum Debtor Optional Redemption Period 3	Jun. 30, 2006 Senior Notes Seven Point One Two Five Percent	Jun. 30, 2008 Senior Notes Seven Point One Two Five Percent	Jun. 30, 2006 Senior Notes Seven Point One Two Five Percent Semi Annual Payment, First Payment	Jun. 30, 2006 Senior Notes Seven Point One Two Five Percent Semi Annual Payment, Second Payment	Nov. 15, 2010 Revolving Credit Facility	Jun. 30, 2012 Revolving Credit Facility Euro Dollar Rate	Nov. 15, 2010 Revolving Credit Facility Letter of Credit	Mar. 15, 2011 Revolving Credit Facility Minimum One Month Eurodollar Rate	Mar. 15, 2011 Revolving Credit Facility Minimum Euro Dollar Rate	Mar. 15, 2011 Revolving Credit Facility Maximum One Month Eurodollar Rate	Mar. 15, 2011 Revolving Credit Facility Maximum Euro Dollar Rate	Nov. 15, 2010 Term Loan A	Jun. 30, 2012 Term Loan A	Jun. 30, 2012 Term Loan A Euro Dollar Rate	Mar. 15, 2011 Term Loan A Minimum One Month Eurodollar Rate	Mar. 15, 2011 Term Loan A Minimum Euro Dollar Rate	Mar. 15, 2011 Term Loan A Maximum One Month Eurodollar Rate	Mar. 15, 2011 Term Loan A Maximum Euro Dollar Rate	Mar. 15, 2011 Term Loan B	Nov. 15, 2010 Term Loan B	Mar. 31, 2011 Term Loan B	Jun. 30, 2012 Term Loan B	Jun. 30, 2012 Term Loan B Euro Dollar Rate	Mar. 15, 2011 Term Loan B Minimum One Month Eurodollar Rate	Mar. 15, 2011 Term Loan B Minimum Euro Dollar Rate	Mar. 15, 2011 Term Loan B Maximum One Month Eurodollar Rate	Mar. 15, 2011 Term Loan B Maximum Euro Dollar Rate	Jun. 30, 2012 Term Loan B Before Amendment Minimum Euro Dollar Rate	Jun. 30, 2012 Term Loan B After Amendment Minimum Euro Dollar Rate	Oct. 31, 2010 Accounts Receivable Securitization Facility	Jun. 30, 2012 Accounts Receivable Securitization Facility	May 31, 2012 Accounts Receivable Securitization Facility	Oct. 31, 2010 Accounts Receivable Securitization Facility Commercial Paper Rate	Oct. 31, 2010 Accounts Receivable Securitization Facility Before Amendment	May 31, 2012 Accounts Receivable Securitization Facility After Amendment	Oct. 31, 2010 Accounts Receivable Securitization Facility After Amendment
Debt Instrument [Line Items]
Senior secured facility borrowing capacity				$ 3,450,000,000																																$ 800,000,000		$ 125,000,000					$ 1,050,000,000								$ 1,600,000,000														$ 200,000,000	$ 275,000,000	$ 240,000,000
Senior secured facility principal payments																																												36,000,000								125,000,000	13,000,000
Line of credit facility, maturity date																																				Nov. 15, 2015							Nov. 15, 2015								Nov. 15, 2016										Oct. 25, 2013
Interest rate															0.50%	1.00%									0.50%																																							0.475%
Consolidated leverage ratio																																							0.50%	1.50%	1.25%	2.25%				0.50%	1.50%	1.25%	2.25%						1.75%	2.75%	2.00%	3.00%
Applicable margin rate																	0.75%																				1.75%								1.75%									2.75%					1.50%	1.00%
Fee paid on amount outstanding																																																		1.00%											0.375%
Available borrowing capacity	623,000,000
Line of credit facility amount outstanding	115,000,000																																																													230,000,000
Letter of credit amount outstanding	62,000,000
Commitment percentage for facility fee																																																													102.00%
Line of credit facility additional borrowing capacity																																																														45,000,000
Senior secured facility borrowing capacity increased																																																															35,000,000
Senior notes, issued																						250,000,000										250,000,000	150,000,000
Senior notes, interest rate																						7.00%										7.125%	7.125%
Senior notes, maturity date																						Oct. 01, 2018										Jun. 30, 2016
Senior notes, frequency of periodic payment																						Semiannually										Semiannually
Senior notes, interest periodic payment																							--04-01	--10-01										--06-30	--12-30
Unsecured Notes make-whole redemption date																									Oct. 01, 2014				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	3,458,509,000	3,651,428,000
Fair value of debt	3,530,000,000
Notional amount of interest rate cash flow hedges					450,000,000	400,000,000	425,000,000	600,000,000	75,000,000		225,000,000	100,000,000	25,000,000	75,000,000
Derivative effective date																		2011-12	2012-12	2012-12	2013-12
Premium paid on interest rate cap							740,000
Derivative cap interest rate			7.00%
Number of interest rate swap							6	4			3
Interest rate maturity								2015-05	2012-10		2015-05	2014-12	2013-12	2012-12
Average fixed interest rate								2.38%	4.76%		1.91%
Fixed interest rate												2.50%	1.96%	1.32%
Fair value of interest rate swap										46,000,000
Fair value of interest rate swap, current										1,000,000
Fair value of interest rate swap, non-current										$ 45,000,000