Related party transactions (Details) (USD $)	3 Months Ended						1 Months Ended			3 Months Ended	1 Months Ended
	May 03, 2013	May 04, 2012	May 03, 2013 Affiliates of KKR and Goldman, Sachs & Co. Term loan facilities	May 04, 2012 Affiliates of KKR and Goldman, Sachs & Co. Term loan facilities	May 03, 2013 Affiliates of KKR and Goldman, Sachs & Co. Revolving credit facilities	May 04, 2012 Affiliates of KKR and Goldman, Sachs & Co. Revolving credit facilities	May 03, 2013 Goldman, Sachs & Co. and affiliates Bank loan facilities	Apr. 06, 2012 Goldman, Sachs & Co. and affiliates Term loan facility	May 03, 2013 Goldman, Sachs & Co. and affiliates Senior notes	May 03, 2013 KKR person	Apr. 06, 2012 KKR Term loan facility	May 03, 2013 KKR Senior notes
Related party transactions
Number of members serving on the entity's board of directors										2
Interest paid			$ 12,000,000	$ 16,100,000	$ 1,100,000	$ 1,000,000
Payment of underwriting fees	$ 15,938,000	$ 7,663,000					$ 700,000	$ 500,000	$ 1,500,000		$ 400,000	$ 700,000