Income Taxes - Reconciliation of income tax computed at the federal statutory rate to the company's effective income tax rate (Details)	12 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2018
Income Tax Disclosure [Abstract]
Tax at U.S. federal statutory rate	21.00%	21.00%	33.80%
Impact of the Tax Cuts and Jobs Act, change in enacted tax rate, percent	0.00%	0.00%	(0.10%)
State and local income taxes, net of U.S. federal benefit	0.80%	1.70%	1.60%
Tax credit for research activities	(4.60%)	(2.30%)	(1.80%)
Tax benefit on qualified production activities	0.00%	0.00%	(3.00%)
Tax Benefit from foreign derived intangible income	(1.10%)	(0.80%)	0.00%
Tax benefit on insurance premiums	(1.20%)	(0.80%)	(1.30%)
Change in uncertain tax positions	0.30%	0.00%	0.10%
Foreign Tax Rate Difference	0.00%	0.10%	0.00%
Impact of settlement of share-based comp awards, percent	(3.30%)	(2.40%)	1.20%
Change in Deferred Tax Assets Valuation Allowance	0.80%	0.00%	0.00%
Other adj, net	0.80%	(0.80%)	0.00%
Effective income tax rate	13.50%	15.70%	30.50%