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Collaboration and Other Agreements, Servier (Details) - USD ($)	1 Months Ended	3 Months Ended								12 Months Ended
	Sep. 30, 2012	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	Jan. 01, 2018	Dec. 31, 2015
Collaboration and Other Agreements [Line Items]
Revenues		$ 15,794,000	$ 20,798,000	$ 18,834,000	$ 4,695,000	$ 152,359,000	$ 1,663,000	$ 1,666,000	$ 2,055,000	$ 60,121,000	$ 157,742,000	$ 91,880,000
Deferred revenue included in current liabilities		21,721,000				7,202,000				21,721,000	7,202,000		$ 7,742,000
Deferred revenue		40,722,000				27,318,000				40,722,000	27,318,000
Increase in revenue recognized from ASC 606 adoption										9,588,000
Servier | Servier DART
Collaboration and Other Agreements [Line Items]
Non-refundable upfront payment	$ 20,000,000.0
Milestone payment removed	5,000,000.0
Option exercise fee														$ 15,000,000.0
Offset to research and development costs under collaboration arrangement										6,000,000.0	3,200,000	2,600,000
Deferred revenue		12,600,000								12,600,000
Deferred revenue, current		900,000								900,000
Deferred revenue, noncurrent		11,700,000								11,700,000
Deferred revenue						7,400,000					7,400,000
Increase in balance related to adoption of ASC 606										6,400,000
Increase in revenue recognized from ASC 606 adoption										100,000
Servier | Maximum | Servier DART
Collaboration and Other Agreements [Line Items]
Potential clinical milestone payments under agreement	22,500,000
Potential regulatory milestone payments under agreement	76,000,000.0
Potential sales milestone payments under agreement	$ 210,000,000.0
Revenues From License Agreements | Servier | Servier DART
Collaboration and Other Agreements [Line Items]
Revenues										1,200,000	1,400,000	2,200,000
Revenues From License Agreements | Servier | Servier Agreement - MGD007 Option
Collaboration and Other Agreements [Line Items]
Revenues										1,900,000	1,100,000	$ 1,100,000
Deferred revenue included in current liabilities						$ 1,100,000					$ 1,100,000
Deferred revenue		$ 0								$ 0

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