Summary of Significant Accounting Policies (Details)	6 Months Ended
Jun. 30, 2020
Maximum
Significant Accounting Policies
Available for sale securities, contractual maturity period (less than)	1 year

Available For Sale Securities Contractual Maturity Period

No definition available.

Name:	alpn_AvailableForSaleSecuritiesContractualMaturityPeriod
Namespace Prefix:	alpn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration