Collaboration and License Revenue (Tables)

12 Months Ended

Dec. 31, 2019

Collaboration And License Agreement Disclosure [Abstract]

Changes in Contract Assets and Liabilities

The following table presents changes in the Company’s contract assets and liabilities, which excludes research and development reimbursements under the cost sharing plan further discussed below, for the year ended December 31, 2018 (in thousands):

		Year Ended December 31, 2018
		Balance at Beginning of Period			Additions		Deductions				Balance at End of Period
Contract liabilities:
Deferred revenue	$		33,558	$	—	$		(33,558	)	$	—

Schedule Prepayments and Reimbursed Research and Development Expenses

The following table presents the Sanofi prepayments and reimbursed research and development expenses for the years ended December 31, 2019, 2018 and 2017 (in thousands):

	Year Ended December 31,
	2019			2018			2017
Balance at beginning of year	$	12,973		$	4,432		$	—
Additions for advance billings		—			—			1,013
Payments received from Sanofi		5,521			31,659			10,697
Actual expenses incurred		(18,494	)		(23,118	)		(7,278	)
Balance at end of year	$	—		$	12,973		$	4,432