Income Taxes - Textual (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Increase (decrease) in valuation allowance	$ 4,800,000	$ (40,500,000)
Research and development credits	46,884,000	43,095,000
Unrecognized tax benefits	21,998,000	$ 20,289,000	$ 18,349,000
Accrued interest or penalties for uncertain income tax	0
U.S. Federal
Income Taxes [Line Items]
Operating loss carryforward	597,000,000
Research and development credits	35,400,000
State
Income Taxes [Line Items]
Operating loss carryforward	189,500,000
California State
Income Taxes [Line Items]
Research and development credits	35,000,000
Other State
Income Taxes [Line Items]
Research and development credits	$ 3,000,000