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

Impairment charge recognized in nonrecurring adjustment for royalty contracts.

No definition available.

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