Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019	Jan. 02, 2017
Summary of Significant Accounting Policies [Line Items]
Number of shares excluded from the computation of net loss per share		59,509,610	46,513,399	39,277,732
Restricted Cash and Cash Equivalents		$ 10,847	$ 28,234
Convertible Notes Initial Conversion Price Per Share		$ 6.81
Foreign currency translation adjustment		$ 11,200	8,600
Adoption of new accounting standard, net of tax			0			$ 600
Subsequent Event [Member]
Summary of Significant Accounting Policies [Line Items]
Lessor, Operating Lease, Term of Contract					12 months
Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-Lived Intangible Asset, Useful Life		7 years
Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-Lived Intangible Asset, Useful Life		20 years
Bill Melinda Gates Foundation [Member]
Summary of Significant Accounting Policies [Line Items]
Grant Agreement	$ 89,100	$ 89,100
Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Summary of Significant Accounting Policies [Line Items]
Operating Lease, Right-of-Use Asset					$ 12,000
Operating Lease, Liability					$ 22,000
Security Deposit [Member]
Summary of Significant Accounting Policies [Line Items]
Restricted Cash and Cash Equivalents		1,000	1,700
Grant Agreement [Member]
Summary of Significant Accounting Policies [Line Items]
Restricted Cash and Cash Equivalents		$ 10,800	$ 27,400
Convertible Debt Securities [Member]
Summary of Significant Accounting Policies [Line Items]
Number of shares excluded from the computation of net loss per share		47,716,900	47,716,900