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Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended			9 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jan. 31, 2020	Sep. 30, 2020	Sep. 30, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average grant date fair value of options granted, per share				$ 1.20	$ 4.71
Grant date fair values of options vested				$ 7.6	$ 11.4
Aggregate intrinsic values of options exercised				0.0	$ 7.8
Unrecognized compensation cost				$ 11.8
Weighted average period for recognition of compensation costs				2 years 1 month 6 days
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted average period for recognition of compensation costs				2 years
Total grant date fair value of RSUs				$ 16.7
Total fair value of RSUs vested				8.1
Unrecognized stock based compensation cost				$ 21.1
Shares expected to vest				8,266,088
Board of Directors \| Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting rights	The June 2020 RSUs granted to board members vest in full on the date of our 2021 Annual Meeting of Stockholders.		The January 2020 RSUs granted to board members vested in full on the date of our 2020 Annual Meeting of Stockholders.
Chief Executive Officer \| Restricted Stock Units (RSUs) \| First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting percentage	42.00%		42.00%
Chief Executive Officer \| Restricted Stock Units (RSUs) \| Second and Third Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting percentage	29.00%		29.00%
Employees \| Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period		3 years	3 years