Income Taxes - Reconciliation of income tax computed at the federal statutory rate to the company's effective income tax rate (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Jan. 31, 2015	Jan. 31, 2014	Jan. 31, 2013
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	(0.30%)	1.50%	1.60%
Tax credit for research activities	(3.90%)	(1.20%)	(0.90%)
Tax benefit on qualified production activities	(3.60%)	(2.90%)	(3.20%)
Other, net	(0.30%)	0.20%	(0.20%)
Effective income tax rate	26.90%	32.60%	32.30%
Favorable prior year tax benefit	$ 776
Discrete tax benefit	$ 963