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, 2018	Jan. 31, 2017	Jan. 31, 2016
Income Tax Disclosure [Abstract]
Tax at U.S. federal statutory rate	33.80%	35.00%	35.00%
Impact of the Tax Cuts and Jobs Act, change in enacted tax rate, percent	(0.10%)	0.00%	0.00%
State and local income taxes, net of U.S. federal benefit	1.60%	0.70%	(2.80%)
Tax credit for research activities	(1.80%)	(3.70%)	(24.20%)
Tax benefit on qualified production activities	(3.00%)	(2.80%)	(13.70%)
Effective income tax rate reconciliation, tax benefit from insurance premium	(1.30%)	(1.50%)	(10.30%)
Effective income tax rate reconciliation, change in uncertain tax positions, percent	0.10%	(0.30%)	1.80%
Effective income tax rate reconciliation, foreign income tax rate differential, percent	0.00%	(0.30%)	(2.90%)
Effective income tax rate reconciliation, impact of settlement of share-based comp awards, percent	1.20%	0.00%	0.00%
Effective income tax rate reconciliation, other adjustments, percent	0.00%	0.40%	(1.70%)
Effective income tax rate	30.50%	27.50%	(18.80%)