Debt Instrument, convertible, initial conversion price.

No definition available.

Name:	apls_DebtInstrumentConvertibleInitialConversionPrice
Namespace Prefix:	apls_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt instrument number of counterparties.

+ References

No definition available.

+ Details

Name:	apls_DebtInstrumentNumberOfCounterparties
Namespace Prefix:	apls_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Nature of organization and operations.

+ References

No definition available.

+ Details

Name:	apls_NatureOfOrganizationAndOperationsLineItems
Namespace Prefix:	apls_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Payments to acquire convertible debt.

+ References

No definition available.

+ Details

Name:	apls_PaymentsToAcquireConvertibleDebt
Namespace Prefix:	apls_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Upfront payment.

+ References

No definition available.

+ Details

Name:	apls_UpfrontPayment
Namespace Prefix:	apls_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3044-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121566466&loc=d3e6676-107765

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

- Definition

Description of the frequency of periodic payments (monthly, quarterly, annual).

+ References

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the payment terms of the debt instrument (for example, whether periodic payments include principal and frequency of payments) and discussion about any contingencies associated with the payment.

+ References

+ Details

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

- Definition

Specific incremental costs directly attributable to a proposed or actual offering of securities which are deferred at the end of the reporting period.

+ References

+ Details

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

- Definition

Amount of investment in marketable security, classified as current.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121566466&loc=d3e6676-107765

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Cash paid for expenses incurred during underwriting activities (the process to review insurance applications, evaluate risks, accept or reject applications, and determine the premiums to be charged) for insurance companies.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3536-108585

+ Details

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

- Definition

The cash outflow paid to third parties in connection with debt origination, which will be amortized over the remaining maturity period of the associated long-term debt.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3291-108585

+ Details

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

- Definition

The cash inflow from the issuance of a long-term debt instrument which can be exchanged for a specified amount of another security, typically the entity's common stock, at the option of the issuer or the holder.

+ 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_ProceedsFromConvertibleDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash inflow from the additional capital contribution to the entity.

+ 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 (a)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

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

- Definition

Per share or per unit amount of equity securities issued.

+ References

No definition available.

+ Details

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

- Definition

Number of new stock issued during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesNewIssues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=apls_CollaborationAndLicenseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=apls_SwedishOrphanBiovitrumABPublMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=apls_FollowOnPublicOfferingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=us-gaap_OverAllotmentOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=us-gaap_PrivatePlacementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

Nature of Organization and Operations - Additional Information (Detail) $ / shares in Units, $ in Thousands						9 Months Ended
	Nov. 02, 2020 USD ($)	Oct. 27, 2020 USD ($)	May 12, 2020 USD ($)	Jan. 13, 2020 USD ($) $ / shares shares	Sep. 16, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	May 06, 2020 CounterParty $ / shares	Dec. 31, 2019 USD ($)	Sep. 11, 2019 CounterParty $ / shares
Nature Of Organization And Operations [Line Items]
Proceeds from follow-on public offering after deducting underwriting discounts and commissions						$ 381,423	$ 109,629
Cash and cash equivalents						415,560	$ 433,995		$ 351,985
Marketable securities						$ 312,598
Subsequent Event [Member] \| Collaboration and License Agreement [Member] \| Swedish Orphan Biovitrum AB (Publ) [Member]
Nature Of Organization And Operations [Line Items]
Upfront Payment	$ 250,000	$ 250,000
Capped Call Transactions [Member]
Nature Of Organization And Operations [Line Items]
Number of counterparties \| CounterParty								2		2
Initial conversion price \| $ / shares								$ 39.4625		$ 39.4625
Follow-on Public Offerings [Member]
Nature Of Organization And Operations [Line Items]
Issuance of common stock in follow-on offering, shares \| shares				10,925,000
Price of common stock \| $ / shares				$ 37.00
Proceeds from follow-on public offering after deducting underwriting discounts and commissions				$ 381,500
Underwriting discounts and commissions				22,200
Offering Cost				$ 500
Over Allotment Option [Member]
Nature Of Organization And Operations [Line Items]
Issuance of common stock in follow-on offering, shares \| shares				1,425,000
Price of common stock \| $ / shares				$ 37.00
Convertible Senior Notes Due 2026 [Member]
Nature Of Organization And Operations [Line Items]
Debt instrument, stated percentage			3.50%
Debt instrument, due and payment description						The 2020 Convertible Notes are senior unsecured obligations of the Company and bear interest at a rate of 3.5% per year payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2020.
Debt instrument, frequency of periodic payment						semiannually
Maturity date			Sep. 15, 2026
Convertible Senior Notes Due 2026 [Member] \| 2020 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Debt instrument, stated percentage			3.50%
Purchase price of convertible notes			$ 328,900
Net proceeds from the sale of the notes			322,900
Payments of transactions cost			43,100
Convertible Senior Notes Due 2026 [Member] \| Private Offering [Member] \| 2020 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Principal amount			300,000
Convertible Senior Notes Due 2026 [Member]
Nature Of Organization And Operations [Line Items]
Principal amount						$ 353,800
Debt instrument, stated percentage					3.50%
Debt instrument, due and payment description						The Convertible Notes are senior unsecured obligations of the Company and bear interest at a rate of 3.5% per year payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2020.
Debt instrument, frequency of periodic payment						semiannually
Maturity date					Sep. 15, 2026
Convertible Senior Notes Due 2026 [Member] \| 2020 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Net proceeds from the sale of the notes			322,900
Payments of transactions cost			43,100
Convertible Senior Notes Due 2026 [Member] \| 2019 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Debt instrument, stated percentage					3.50%
Net proceeds from the sale of the notes					$ 212,900
Payments of transactions cost					28,400
Convertible Senior Notes Due 2026 [Member] \| Private Offering [Member] \| 2020 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Principal amount			$ 300,000
Convertible Senior Notes Due 2026 [Member] \| Private Offering [Member] \| 2019 Convertible Notes [Member]
Nature Of Organization And Operations [Line Items]
Principal amount					$ 220,000