Class of Warrant or Right, Number Of Shares Exercisable

No definition available.

Name:	alpn_ClassOfWarrantOrRightNumberOfSharesExercisable
Namespace Prefix:	alpn_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Aggregate New Capital Milestone For Continuation Of Interest Only Payment Period

+ References

No definition available.

+ Details

Name:	alpn_DebtInstrumentAggregateNewCapitalMilestoneForContinuationOfInterestOnlyPaymentPeriod
Namespace Prefix:	alpn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt instrument final payment percentage.

+ References

No definition available.

+ Details

Name:	alpn_DebtInstrumentFinalPaymentPercentage
Namespace Prefix:	alpn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Interest Rate Floor

+ References

No definition available.

+ Details

Name:	alpn_DebtInstrumentInterestRateFloor
Namespace Prefix:	alpn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Maximum Borrowing Capacity, Amount

+ References

No definition available.

+ Details

Name:	alpn_DebtInstrumentMaximumBorrowingCapacityAmount
Namespace Prefix:	alpn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Debt Instrument, Number Of Equal Monthly Installments Payable

+ References

No definition available.

+ Details

Name:	alpn_DebtInstrumentNumberOfEqualMonthlyInstallmentsPayable
Namespace Prefix:	alpn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Percentage Change in Non-Troubled Debt Restructuring Based On Comparison Of Present Value Of Cash Flows

+ References

No definition available.

+ Details

Name:	alpn_DebtInstrumentPercentageChangeinNonTroubledDebtRestructuringBasedOnComparisonOfPresentValueOfCashFlows
Namespace Prefix:	alpn_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Percentage Of Default Interest Rate

+ References

No definition available.

+ Details

Name:	alpn_DebtInstrumentPercentageOfDefaultInterestRate
Namespace Prefix:	alpn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Prepayment Fee, Percentage

+ References

No definition available.

+ Details

Name:	alpn_DebtInstrumentPrepaymentFeePercentage
Namespace Prefix:	alpn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Prepayment Fee Representing Deferred Portion Of Final Payment

+ References

No definition available.

+ Details

Name:	alpn_DebtInstrumentPrepaymentFeeRepresentingDeferredPortionOfFinalPayment
Namespace Prefix:	alpn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt Instrument, Total Discount Recorded

+ References

No definition available.

+ Details

Name:	alpn_DebtInstrumentTotalDiscountRecorded
Namespace Prefix:	alpn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt Instrument, Unused Borrowing Capacity, Expired Amount

+ References

No definition available.

+ Details

Name:	alpn_DebtInstrumentUnusedBorrowingCapacityExpiredAmount
Namespace Prefix:	alpn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Dent Instrument, Number Of Tranches

+ References

No definition available.

+ Details

Name:	alpn_DentInstrumentNumberOfTranches
Namespace Prefix:	alpn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28541-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

Name:	us-gaap_AmortizationOfDebtDiscountPremium
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Exercise price per share or per unit of warrants or rights outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

+ Details

Name:	us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares.

+ References

+ Details

Name:	us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt discount.

+ References

+ Details

Name:	us-gaap_DebtInstrumentUnamortizedDiscount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=121570589&loc=SL51823488-111719

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624163-113959

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfLongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3291-108585

+ Details

Name:	us-gaap_RepaymentsOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_LineOfCreditFacilityAxis=alpn_SiliconValleyBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=alpn_OriginalAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=alpn_CommonStockWarrantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_PrimeRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=alpn_TermLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=alpn_TermLoansTrancheOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=alpn_TermLoansTrancheTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_BaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=alpn_TermLoansTrancheThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=us-gaap_DebtInstrumentRedemptionPeriodOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=us-gaap_DebtInstrumentRedemptionPeriodTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=us-gaap_DebtInstrumentRedemptionPeriodThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-term Debt - Narrative (Details)	1 Months Ended			3 Months Ended		6 Months Ended
Long-term Debt - Narrative (Details)	Mar. 31, 2020 USD ($) shares	Aug. 31, 2019 USD ($) installment tranche $ / shares shares	Dec. 31, 2016 USD ($) $ / shares	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jul. 24, 2017 shares
Debt Instrument [Line Items]
Proceeds from borrowings, net of issuance costs						$ 5,000,000	$ 0
Amortization of debt discount				$ 81,000	$ 31,000	125,000	$ 67,000
Unamortized debt discount				632,000		632,000
Silicon Valley Bank \| Original Agreement
Debt Instrument [Line Items]
Proceeds from borrowings, net of issuance costs			$ 5,000,000.0
Repayments of debt		$ 3,000,000.0
Prepayment fee (equal to)		75,000
Silicon Valley Bank \| Original Agreement \| Common Stock Warrant
Debt Instrument [Line Items]
Warrants issued for shares of common stock (in shares) \| shares								7,069
Warrants issued, exercise price (in dollars per share) \| $ / shares			$ 12.38
Silicon Valley Bank \| Original Agreement \| Prime Rate
Debt Instrument [Line Items]
Debt instrument variable interest rate			1.75%
Term Loans
Debt Instrument [Line Items]
Debt instrument, total discount recorded				$ 812,000		$ 812,000
Term Loans \| Term Loans, Tranche One \| Common Stock Warrant
Debt Instrument [Line Items]
Fair value of warrant		$ 60,000
Term Loans \| Term Loans, Tranche Two \| Common Stock Warrant
Debt Instrument [Line Items]
Fair value of warrant	$ 60,000
Term Loans \| Silicon Valley Bank
Debt Instrument [Line Items]
Debt instrument variable interest rate		0.25%
Maximum borrowing capacity, amount		$ 15,000,000.0
Number of tranches (up to) \| tranche		3
Percentage of default interest rate		4.00%
New capital milestone for continuation of interest only payment period		$ 40,000,000.0
Number of equal monthly installments payable \| installment		25
Final payment percentage		5.50%
Term Loans \| Silicon Valley Bank \| Common Stock Warrant
Debt Instrument [Line Items]
Warrants issued for shares of common stock (in shares) \| shares		52,083
Warrants issued, exercise price (in dollars per share) \| $ / shares		$ 4.32
Number of shares exercisable (in shares) \| shares	17,361	17,361
Term Loans \| Silicon Valley Bank \| Base Rate
Debt Instrument [Line Items]
Debt instrument, interest rate floor		5.75%
Term Loans \| Silicon Valley Bank \| Term Loans, Tranche One
Debt Instrument [Line Items]
Proceeds from borrowings, net of issuance costs		$ 5,000,000.0
Term Loans \| Silicon Valley Bank \| Term Loans, Tranche Two
Debt Instrument [Line Items]
Proceeds from borrowings, net of issuance costs	$ 5,000,000.0
Term Loans \| Silicon Valley Bank \| Term Loans, Tranche Three
Debt Instrument [Line Items]
Unused borrowing capacity, expired amount		$ 5,000,000.0
Debt Instrument, Redemption, Period One \| Term Loans \| Silicon Valley Bank
Debt Instrument [Line Items]
Redemption price, percentage		2.00%
Debt Instrument, Redemption, Period Two \| Term Loans \| Silicon Valley Bank
Debt Instrument [Line Items]
Redemption price, percentage		1.00%
Debt Instrument, Redemption, Period Three \| Term Loans \| Silicon Valley Bank
Debt Instrument [Line Items]
Redemption price, percentage		0.00%
Maximum
Debt Instrument [Line Items]
Percentage change in loan in a non-troubled debt restructuring (less than)		0.10