Fair Value Measurements - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Jul. 26, 2018	Aug. 31, 2017
3.25% Convertible Senior Notes (due 2020)
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Debt instrument, interest rate	3.25%	3.25%	3.25%	3.25%
3.25% convertible senior notes due 2023
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Debt instrument, interest rate	3.25%
Accrued paid-in-kind interest	$ 100
Level 1 | 3.25% Convertible Senior Notes (due 2020) | Fair Value Measurements, Recurring
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	0	$ 0
Level 1 | 3.25% convertible senior notes due 2023 | Fair Value Measurements, Recurring
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	0
Level 2 | 3.25% Convertible Senior Notes (due 2020) | Fair Value Measurements, Recurring
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	0	0
Level 2 | 3.25% convertible senior notes due 2023 | Fair Value Measurements, Recurring
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	0
Level 3 | 3.25% Convertible Senior Notes (due 2020) | Fair Value Measurements, Recurring
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	0	27,974
Level 3 | 3.25% convertible senior notes due 2023 | Fair Value Measurements, Recurring
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	48,635
Carrying Amount | 3.25% Convertible Senior Notes (due 2020) | Fair Value Measurements, Recurring
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	0	$ 27,974
Carrying Amount | 3.25% convertible senior notes due 2023 | Fair Value Measurements, Recurring
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	48,635
Redeemable Preferred Stock | Level 1 | Fair Value Measurements, Recurring
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	0
Redeemable Preferred Stock | Level 2 | Fair Value Measurements, Recurring
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	0
Redeemable Preferred Stock | Level 3 | Fair Value Measurements, Recurring
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	4,894
Redeemable Preferred Stock | Carrying Amount | Fair Value Measurements, Recurring
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	$ 4,894