Recent accounting pronouncements (Details) - USD ($) $ in Thousands	12 Months Ended
	Jan. 31, 2017	Jan. 31, 2016	Jan. 31, 2015
New Accounting Pronouncement, Early Adoption [Line Items]
Deferred Tax Assets, Gross	$ 179,443	$ 162,394
Deferred tax assets valuation allowance	74,041	63,554
Cumulative Effect of New Accounting Principle in Period of Adoption		10,180
Net Cash Provided by Operating Activities	321,811	231,250	$ 140,839
Net Cash Used in Financing Activities	$ (145,323)	(72,561)	(71,685)
Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
Deferred Tax Assets, Gross		48,605
Deferred tax assets valuation allowance		38,425
Cumulative Effect of New Accounting Principle in Period of Adoption		10,180
Scenario, Previously Reported [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
Net Cash Provided by Operating Activities		228,596	138,208
Net Cash Used in Financing Activities		$ (69,907)	$ (69,054)