Quarterly Results (unaudited) - Summary of Unaudited Quarterly Results (Detail) - USD ($)	3 Months Ended	12 Months Ended
Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
License revenue, and other	$ 2,144	$ 18,821,953	$ 1,333,333						$ 20,157,430
Type of Revenue [Extensible List]	arpo:LicenseRevenueAndOtherMember	arpo:LicenseRevenueAndOtherMember	arpo:LicenseRevenueAndOtherMember						arpo:LicenseRevenueAndOtherMember
Total operating expenses	$ 8,868,322	$ 7,623,916	$ 7,369,788	$ 7,476,648	$ 6,288,858	$ 4,756,238	$ 5,583,862	$ 4,759,585	$ 31,338,674	$ 21,388,543
Loss from operations	(8,866,178)	11,198,037	(6,036,455)	(7,476,648)	(6,288,858)	(4,756,238)	(5,583,862)	(4,759,585)	(11,181,244)	(21,388,543)
Other (expense) income, net	347,588	339,441	46,464	51,116	53,829	106,671	63,555	(236,118)	784,609	(12,063)
Net and comprehensive loss					(6,235,029)	(4,649,567)	(5,520,307)	(4,995,703)	(10,396,635)	(21,400,606)
Reconciliation of net loss attributable to common stockholders:
Net and comprehensive loss					(6,235,029)	(4,649,567)	(5,520,307)	(4,995,703)	(10,396,635)	(21,400,606)
Adjustment of redeemable convertible preferred stock to redemption value								(943,297)		(943,297)
Net loss attributable to common stockholders	$ (8,518,590)	$ 11,537,478	$ (5,989,991)	$ (7,425,532)	$ (6,235,029)	$ (4,649,567)	$ (5,520,307)	$ (5,939,000)	$ (10,396,635)	$ (22,343,903)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.21)	$ 0.28	$ (0.22)	$ (0.27)	$ (0.23)	$ (0.17)	$ (0.21)	$ (1.06)	$ (0.31)	$ (1.03)
Weighted average number of common shares used in computing net (loss) income per share:
Basic	40,587,928	40,527,722	27,340,914	27,045,509
Diluted	40,587,928	40,961,620	27,340,914	27,045,509
Weighted average number of common shares used in computing net loss per share attributable to common stockholders, basic and diluted					26,965,293	26,926,673	26,895,164	5,605,151	33,930,846	21,673,349

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The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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