Summary of Significant Accounting Policies - Schedule of Assets and Liabilities Carried at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	$ 759,257	$ 450,230
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	759,257	450,230
Available-for-sale securities		327,455
Restricted cash	4,322	2,160
Derivative Liability	619
Redemption Note		1,879,058
Fair Value, Measurements, Recurring | Quoted Market Prices in Active Markets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	0	11,200
Available-for-sale securities		0
Restricted cash	2,015	0
Derivative Liability	0
Redemption Note		0
Fair Value, Measurements, Recurring | Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	759,257	439,030
Available-for-sale securities		327,455
Restricted cash	2,307	2,160
Derivative Liability	619
Redemption Note		1,879,058
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Cash equivalents	0	0
Available-for-sale securities		0
Restricted cash	0	0
Derivative Liability	$ 0
Redemption Note		$ 0