Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Line Items]
Unrecognized tax benefits	$ 0	$ 0
Accrual for interest or penalties	0	0
Recognized interest and penalties	0	$ 0	$ 0
Domestic Tax Authority [Member]
Income Tax Disclosure [Line Items]
Income tax net operating loss carryforwards	537,100,000
Domestic Tax Authority [Member] | Research Tax Credit Carryforward [Member]
Income Tax Disclosure [Line Items]
California research and development tax credit carryforwards	21,000,000
State and Local Jurisdiction [Member]
Income Tax Disclosure [Line Items]
Income tax net operating loss carryforwards	323,500,000
State and Local Jurisdiction [Member] | Research Tax Credit Carryforward [Member]
Income Tax Disclosure [Line Items]
California research and development tax credit carryforwards	$ 13,400,000