Development and License Agreements - Serum License Agreement (Details) - USD ($)	1 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2019	Jul. 31, 2019	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Collaborative Arrangements and Non-collaborative Arrangement Transactions
Issuance of common stock in private placement, net of issuance costs			$ 4,958,000		$ 4,958,000
Deferred revenue, current				$ 14,680,000		$ 14,602,000
Deferred revenue, noncurrent				4,922,000		5,000,000
Capitalized contract cost, Current				1,545,000		1,537,000
Capitalized contract cost, Noncurrent				518,000		526,000
Licenses and know-how
Collaborative Arrangements and Non-collaborative Arrangement Transactions
Deferred revenue				14,500,000
Development support services
Collaborative Arrangements and Non-collaborative Arrangement Transactions
Deferred revenue				79,000
Research and Development Option
Collaborative Arrangements and Non-collaborative Arrangement Transactions
Deferred revenue				892,000
Manufacturing rights option
Collaborative Arrangements and Non-collaborative Arrangement Transactions
Deferred revenue				4,100,000
License Agreement with SIBV
Collaborative Arrangements and Non-collaborative Arrangement Transactions
Upfront payment received	$ 15,000,000	$ 5,000,000
License Agreement with SAMR
Collaborative Arrangements and Non-collaborative Arrangement Transactions
Buy back manufacturing rights	$ 5,000,000
Maximum additional payments entitled				42,500,000
License Agreement and Option Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions
Deferred revenue				19,600,000
Deferred revenue based on upfront payments				15,000,000
Deferred revenue from equity allocation				4,600,000
Serum International B.V. ("SIBV")
Collaborative Arrangements and Non-collaborative Arrangement Transactions
Issuance of common stock in private placement, net of issuance costs (in shares)		801,820
Issuance of common stock in private placement, net of issuance costs		$ 10,000,000
Deferred revenue				19,600,000
Serum International B.V. ("SIBV") | License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions
Capitalized contract costs				2,100,000		2,100,000
Deferred revenue, current				14,700,000		14,600,000
Deferred revenue, noncurrent				4,900,000		5,000,000
Capitalized incremental costs of obtaining the License Agreement				2,100,000		2,100,000
Capitalized contract cost, Current				1,500,000		1,500,000
Capitalized contract cost, Noncurrent				518,000		526,000
Serum International B.V. ("SIBV") | License Agreement with SIBV
Collaborative Arrangements and Non-collaborative Arrangement Transactions
Deferred revenue				19,600,000
Deferred revenue based on upfront payments				15,000,000
Deferred revenue from equity allocation				4,600,000
Issuance costs from equity allocation				4,600,000
Deferred revenue, noncurrent						5,000,000
Capitalized contract cost, Noncurrent						$ 526,000
Restricted common stock | Serum International B.V. ("SIBV")
Collaborative Arrangements and Non-collaborative Arrangement Transactions
Issuance of common stock in private placement, net of issuance costs (in shares)		801,820
Issuance of common stock in private placement, net of issuance costs		$ 10,000,000
Proceeds from sale of restricted common stock and upfront payment				5,400,000
Fair value of net proceeds				441,000
Payments of Stock Issuance Costs				$ 5,000,000