Organization and Significant Accounting Policies - Additional Information (Detail)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) Obligation	Sep. 30, 2018 USD ($) Obligation	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit incurred	$ (714,403,000)	$ (714,403,000)	$ (646,081,000)
Cash requirements term		12 months
Effective tax rate		21.00%	34.00%
Provision for income taxes	$ 0	$ 0
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of performance obligation | Obligation	1	1
Accounting Standards Update 2014-09 [Member] | Difference Between Revenue Guidance in Effect Before and After Topic 606 [Member]
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit incurred				$ (11,500,000)
Accounting Standards Update 2016-02 [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating leases with lease terms	12 months	12 months