Represents the number of additional options to extend the lease term.

No definition available.

Name:	ontx_OperatingLeasesNumberOfAdditionalOptionsForLeaseExtension
Namespace Prefix:	ontx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the amount of payments that the lessee is obligated to make or can be required to make monthly in connection with a property under the terms of an agreement classified as an operating lease, excluding contingent rentals and a guarantee by the lessee of the lessor's debt and the lessee's obligation to pay (apart from the rental payments) executory costs such as insurance, maintenance, and taxes.

+ References

No definition available.

+ Details

Name:	ontx_OperatingLeasesRentExpenseMinimumRentalsPerMonth
Namespace Prefix:	ontx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of payments that the lessee is obligated to make or can be required to make monthly in connection with a property sub-leased under the terms of an agreement classified as an operating lease, excluding contingent rentals and a guarantee by the lessee of the lessor's debt and the lessee's obligation to pay (apart from the rental payments) executory costs such as insurance, maintenance, and taxes.

+ References

No definition available.

+ Details

Name:	ontx_OperatingSubLeasesRentExpensePerMonth
Namespace Prefix:	ontx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseRenewalTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=ontx_OfficeAndLabSpaceOf8100SquareFeetAreaInNewtownPennsylvaniaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=ontx_OfficeSpaceOf1356SquareFeetAreaInNewtownPennsylvaniaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies - Office and Lab Space in Newton, Pennsylvania (Details)	1 Months Ended	12 Months Ended						19 Months Ended
	Oct. 31, 2009 Option	Feb. 28, 2020 USD ($)	Feb. 28, 2019 USD ($)	Feb. 28, 2018 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Feb. 28, 2017 USD ($)	Sep. 30, 2012 ft²	Jan. 31, 2007 ft²
Operating leases
Lease rent per month		$ 14,700	$ 14,200	$ 13,800
Office and lab space of 8,100 square feet area in Newtown, Pennsylvania
Operating leases
Area of space leased (in square feet) \| ft²										8,100
Number of options available for lease extensions \| Option	3
Additional lease term under option	1 year
Lease rent per month					$ 11,900	$ 11,500	$ 11,000
Additional office space of 1,356 square feet area in Newtown, Pennsylvania
Operating leases
Area of space leased (in square feet) \| ft²									1,356
Sub-lease rent per month								$ 1,600