Leases - Lease assets and liabilities (Details) - USD ($) $ in Thousands	Apr. 30, 2020	Jan. 31, 2020	Apr. 30, 2019
Assets and Liabilities, Lessee [Abstract]
Operating lease assets	$ 251,565	$ 270,032	$ 320,169
Classification of operating lease assets	us-gaap:OperatingLeaseRightOfUseAsset
Total lease assets	$ 251,565	270,032	320,169
Current operating lease liabilities	$ 63,632	63,166	74,761
Classification current operating lease liabilities	us-gaap:OperatingLeaseLiabilityCurrent
Noncurrent operating lease liabilities	$ 231,323	249,040	286,663
Classification of noncurrent operating liabilities	us-gaap:OperatingLeaseLiabilityNoncurrent
Total lease liabilities	$ 294,955	$ 312,206	$ 361,424