Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Income Tax Contingency [Line Items]
Income tax provision	$ 2,700,000
Valuation allowance for company		$ 9,000,000	$ 23,000,000
U.S. Federal net operating losses		518,000,000
State net operating losses		270,000,000
U.S. Federal net operating losses, amount limited to 80% of taxable income, carried forward indefinitely		$ 97,000,000
U.S. Federal net operating losses, taxable income threshold		80.00%
U.S. Federal net operating losses, amount not limited to taxable income, carried forward indefinitely		$ 99,000,000
U.S. Federal net operating losses, amount subject to expiration in 2028		$ 319,000,000
Minimum percentage of ownership required to increase in stock of the ownership		5.00%
Increase in ownership percentage of certain stockholders		50.00%
Ownership change, increase in ownership percentage,term		3 years
Unrecognized tax benefits		$ 7,997,000	6,736,000	$ 4,985,000	$ 3,489,000
Unrecognized tax benefits that would affect income tax expense		8,000,000
Accrued interest and penalties related to uncertain tax positions		0	$ 0	$ 0
U.S. | Research and Development
Income Tax Contingency [Line Items]
Research and development credits		17,200,000
California | Research and Development
Income Tax Contingency [Line Items]
Research and development credits		2,800,000
Oregon | Research and Development
Income Tax Contingency [Line Items]
Research and development credits		$ 2,900,000