Assets and liabilities measured at fair value (Details) (USD $) In Thousands, unless otherwise specified	Oct. 31, 2014 Accrued expenses and other current liabilities	Jan. 31, 2014 Noncurrent Other liabilities	Oct. 31, 2014 Reported amount Current portion of long-term debt obligations	Oct. 31, 2014 Reported amount Long-term obligations	Oct. 31, 2014 Reported amount Accrued expenses and other current liabilities	Oct. 31, 2014 Reported amount Noncurrent Other liabilities	Oct. 31, 2014 Fair value measurements on recurring basis Balance at the end of the period	Oct. 31, 2014 Fair value measurements on recurring basis Balance at the end of the period Accrued expenses and other current liabilities	Oct. 31, 2014 Fair value measurements on recurring basis Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1)	Oct. 31, 2014 Fair value measurements on recurring basis Significant Other Observable Inputs (Level 2)
Liabilities:
Long-term obligations			$ 101,041	$ 2,666,022			$ 2,677,522		$ 2,658,100	$ 19,422
Derivative financial instruments	2,195	4,109						2,195		2,195
Deferred compensation					$ 2,038	$ 21,294	$ 23,332		$ 23,332