Segment Information - Reconciliation of U.S. Gaap Income Before Income Taxes to Adjusted EBITDA (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Segment Reporting [Abstract]
Impairment charge recognized in non-recurring adjustment for royalty contracts	$ 0.2	$ 0.9
Income recognized in non-recurring adjustment for change in estimated legal expenses		$ 2.0

Income recognized in non-recurring adjustment for change in estimated legal expenses.

No definition available.

Name:	altr_IncomeRecognizedInNonrecurringAdjustmentForChangeInEstimatedLegalExpenses
Namespace Prefix:	altr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration