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, 2017	Jan. 31, 2016	Jan. 31, 2015
Income Tax Disclosure [Abstract]
Tax at U.S. federal statutory rate	35.00%	35.00%	35.00%
State and local income taxes, net of U.S. federal benefit	0.70%	(2.80%)	(0.30%)
Tax credit for research activities	(3.70%)	(24.20%)	(3.90%)
Tax benefit on qualified production activities	(2.80%)	(13.70%)	(3.60%)
Effective income tax rate reconciliation, Tax benefit from insurance premium	(1.50%)	(10.30%)	(1.00%)
Effective Income Tax Rate Reconciliation, Tax Contingency, Domestic, Percent	(0.30%)	1.80%	0.00%
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent	(0.30%)	(2.90%)	0.40%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.40%	(1.70%)	0.30%
Effective income tax rate	27.50%	(18.80%)	26.90%