Income taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Schedule Components of Loss Before Income Taxes

The components of loss before income taxes were as follows (in thousands):

	Year ended December 31,
	2019		2018		2017
U.S.	$	(649,172)	$	(440,473)	$	(266,236)
Foreign	(140,981)		(114,965)		(69,197)
Total	$	(790,153)	$	(555,438)	$	(335,433)

Summary of Provision for (Benefit from) Income Taxes

The provision for (benefit from) income taxes were as follows (in thousands):

	Year ended December 31,
	2019		2018		2017
Current:
Federal	$	—	$	—	$	—
State	7		324		115
Foreign	612		222		95
Deferred:
Federal	(966)		(307)		—
State	(198)		(52)		—
Foreign	—		—		—
Total income tax (benefit) expense	$	(545)	$	187	$	210

Reconciliation of Income Tax Provision (Benefit) Computed at the Statutory Federal Income Tax Rate to Effective Income Tax Rate (Provision) Benefit as Reflected in the Financial Statements

A reconciliation of income tax provision (benefit) computed at the statutory federal income tax rate to the Company’s effective income tax rate (provision) benefit as reflected in the financial statements is as follows:

	Year ended December 31,
	2019		2018		2017
Federal income tax expense at statutory rate	21.0	%	21.0	%	34.0	%
State income tax, net of federal benefit	3.7	%	5.1	%	4.3	%
Permanent differences	(1.1)	%	0.9	%	2.7	%
Research and development credit	5.4	%	6.5	%	12.8	%
Foreign differential	(3.7)	%	(4.4)	%	(6.9)	%
Federal tax rate change	—	%	0.1	%	(31.6)	%
Other	0.4	%	(0.1)	%	(0.8)	%
Change in valuation allowance	(25.6)	%	(29.1)	%	(14.6)	%
Effective income tax rate benefit (expense)	0.1	%	—	%	(0.1)	%

Components of Deferred Tax Assets and Liabilities

The significant components of the Company’s deferred tax assets and liabilities are composed of the following (in thousands):

	Year ended December 31,
	2019		2018
Deferred tax assets:
U.S. net operating loss carryforwards (federal and state)	$	439,839	$	298,701
Tax credit carryforwards (federal and state)	213,810		167,517
Capitalized license fees and research and development expenses	16,295		18,083
60 Binney Street lease	—		42,059
Deferred revenue	15,119		21,442
Stock-based compensation	46,111		31,858
Lease liabilities	52,631		—
Accruals and other	15,432		8,886
Total deferred tax assets	799,237		588,546
Intangible assets	(2,518)		(3,579)
Right-of-use assets	(49,480)		—
Fixed assets	(2,670)		(41,472)
Less valuation allowance	(744,569)		(543,495)
Net deferred taxes	$	—	$	—

Reconciliation of Unrecognized Tax Benefits

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands):

	Unrecognized tax benefits
Balance as of December 31, 2017	$	—
Increases for tax positions related to current period	3,370
Increases for tax positions related to prior periods	8,725
Balance as of December 31, 2018	12,095
Increases for tax positions related to current period	3,554
Increases for tax positions related to prior periods	296
Balance as of December 31, 2019	15,945