NET INCOME PER SHARE - Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	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	Dec. 31, 2016	Aug. 07, 2017
Numerator:
Net income attributable to Innoviva stockholders, basic	$ 263,760	$ 47,089	$ 54,626	$ 29,581	$ 58,385	$ 23,767	$ 35,146	$ 16,845	$ 395,056	$ 134,143	$ 59,536
Add: interest expense on 2023 Notes									5,661	5,647	5,790
Net income attributable to Innoviva stockholders, diluted									$ 400,717	$ 139,790	$ 65,326
Denominator:
Weighted-average shares used to compute basic net income per share attributable to Innoviva stockholders									100,849	106,945	110,280
Dilutive effect of 2023 Notes									12,189	12,189	12,541
Dilutive effect of options and awards granted under equity incentive plan and employee stock purchase plan									370	732	412
Weighted-average shares used to compute diluted net income per share attributable to Innoviva stockholders									113,408	119,866	123,233
Net income per share attributable to Innoviva stockholders
Basic net income per share	$ 2.61	$ 0.47	$ 0.54	$ 0.29	$ 0.55	$ 0.22	$ 0.33	$ 0.16	$ 3.92	$ 1.25	$ 0.54
Diluted net income per share	2.34	$ 0.43	$ 0.49	$ 0.27	$ 0.50	$ 0.21	$ 0.30	$ 0.15	$ 3.53	$ 1.17	$ 0.53
Convertible senior notes | 2025 Notes
Net Income Per Share
Dilutive effect of 2025 Notes									0	0
Convertible senior notes | 2025 Notes | Common Stock
Net Income Per Share
Conversion price of convertible notes into common stock (in dollars per share)	$ 17.26								$ 17.26			$ 17.26