License, Research and Other Agreements (Narrative) (Details) - USD ($)			12 Months Ended
	Jan. 25, 2016	Dec. 05, 2014	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred Revenue Arrangement [Line Items]
Estimate of total payments for clinical trials			$ 319,800,000
Clinical trials expense			87,700,000	$ 41,500,000	$ 35,800,000
Cumulative payments for clinical trials			239,600,000
Research and Development Expense [Member]
Deferred Revenue Arrangement [Line Items]
Clinical trials expense			254,000,000
UVA license agreement [Member] | Maximum [Member]
Deferred Revenue Arrangement [Line Items]
Milestone payments for license costs			2,700,000
UVA license agreement [Member] | Minimum [Member]
Deferred Revenue Arrangement [Line Items]
Penalties for Milestone Payment			150,000
LICR [Member]
Deferred Revenue Arrangement [Line Items]
License costs		$ 1,000,000	$ 0	$ 0	$ 2,000,000
Type of Cost, Good or Service [Extensible List]		us-gaap:LicenseMember			us-gaap:LicenseMember
Milestone payments for license costs pre-regulatory approval	$ 12,000,000	$ 20,000,000
Milestone payments for license costs post-regulatory approval	$ 32,000,000	$ 80,000,000