Consolidated statement of changes in stockholders' equity (Parenthetical) $ in Thousands	3 Months Ended
Jun. 30, 2018 USD ($)
Statement Of Stockholders Equity [Abstract]
Follow-on public offering, offering costs	$ 370

Offering cost of common stock in connection with follow-on public offering.

No definition available.

Name:	altr_OfferingCostOfCommonStockInConnectionWithFollowOnPublicOffering
Namespace Prefix:	altr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration