Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)		3 Months Ended								12 Months Ended			61 Months Ended
	Jan. 01, 2018	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	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Net loss		$ (4,666,000)	$ 6,115,000	$ (3,976,000)	$ (5,574,000)	$ 9,463,000	$ (10,692,000)	$ (15,225,000)	$ (22,608,000)	$ (8,101,000)	$ (39,062,000)	$ (103,102,000)
Net loss per common share, basic and diluted		$ (0.12)	$ 0.16	$ (0.10)	$ (0.15)	$ 0.25	$ (0.28)	$ (0.40)	$ (0.61)	$ (0.21)	$ (1.04)	$ (3.14)
Impairment of long-lived assets										$ 0
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued		$ 0				$ 86,000				0	$ 86,000		$ 0
Contract liability		1,100,000								1,100,000			1,100,000
Decrease in deferred revenue										44,421,000	36,045,000	$ 21,272,000
Collaboration revenue		10,184,000	$ 19,518,000	$ 6,870,000	$ 7,849,000	20,640,000	$ 5,106,000	$ 6,195,000	$ 6,213,000	44,421,000	38,154,000	25,153,000
ASU 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Decrease in accumulated deficit	$ 98,300,000
Decrease in deferred revenue										142,700,000
ASU 2014-09 [Member] \| Topic 605 [Member]
Summary Of Significant Accounting Policies [Line Items]
Net loss										50,220,000
ASU 2014-09 [Member] \| Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Net loss										58,321,000
Contract liability	45,500,000
Decrease in accumulated deficit	98,300,000
Decrease in deferred revenue	98,300,000
Collaboration Revenue [Member]
Summary Of Significant Accounting Policies [Line Items]
Collaboration revenue										44,421,000	36,016,000	21,277,000
Collaboration Revenue [Member] \| ASU 2014-09 [Member] \| Topic 605 [Member]
Summary Of Significant Accounting Policies [Line Items]
Collaboration revenue										102,742,000
Collaboration Revenue [Member] \| ASU 2014-09 [Member] \| Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Collaboration revenue										58,321,000
Collaborative Arrangement [Member] \| Collaboration Revenue [Member]
Summary Of Significant Accounting Policies [Line Items]
Collaboration revenue										44,421,000	38,154,000	25,153,000
Celgene [Member] \| Collaborative Arrangement [Member]
Summary Of Significant Accounting Policies [Line Items]
Contract liability		$ 1,100,000				143,800,000				1,100,000	143,800,000		1,100,000
Celgene [Member] \| Collaborative Arrangement [Member] \| Topic 605 [Member]
Summary Of Significant Accounting Policies [Line Items]
Contract liability						$ 143,800,000					143,800,000
Celgene [Member] \| Collaborative Arrangement [Member] \| ASU 2014-09 [Member] \| Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Net loss										$ 7,700,000
Net loss per common share, basic and diluted										$ 0.20
Contract liability	45,500,000
Decrease in accumulated deficit	$ 98,300,000
Celgene [Member] \| Collaborative Arrangement [Member] \| Collaboration Revenue [Member]
Summary Of Significant Accounting Policies [Line Items]
Collaboration revenue										$ 44,421,000	$ 35,997,000	$ 21,833,000
Celgene [Member] \| Collaborative Arrangement [Member] \| Collaboration Revenue [Member] \| ASU 2014-09 [Member] \| Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Collaboration revenue													$ 186,200,000
Revenue [Member] \| Customer Concentration [Member] \| Celgene [Member] \| Collaborative Arrangement [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk percentage of revenue represented by major customers										100.00%	94.00%	87.00%
Leasehold Improvements [Member]
Summary Of Significant Accounting Policies [Line Items]
Description of property and equipment over estimated useful lives										shorter of their estimated useful lives or the remaining life of the lease at the time the asset is placed into service
Leasehold Improvements [Member] \| Change in Estimated Useful Life of Property and Equipment Accounted for as Change in Accounting Estimate [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting estimate										Company changed the estimated useful life of its leasehold improvements to better reflect the period during which these assets are expected to remain in service. The change in estimated useful life has been accounted for as a change in accounting estimate. The remaining carrying amounts of the leasehold improvements as of December 31, 2018 will be amortized prospectively over one year up to December 31, 2019.
Leasehold Improvements [Member] \| Change in Estimated Useful Life of Property and Equipment Accounted for as Change in Accounting Estimate [Member] \| Restatement Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, Estimated useful life										1 year
Net loss										$ 26,000
Maximum [Member] \| Property and Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, Estimated useful life										5 years
Maximum [Member] \| Leasehold Improvements [Member] \| Change in Estimated Useful Life of Property and Equipment Accounted for as Change in Accounting Estimate [Member] \| Restatement Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Net loss per common share, basic and diluted										$ 0.01
Minimum [Member] \| Revenue [Member] \| Customer Concentration [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk percentage of revenue represented by major customers										10.00%
Minimum [Member] \| Property and Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, Estimated useful life										3 years

OncoMed Pharmaceuticals Inc 10-K