Fair Value - Summary of Fair Value Hierarchy of Assets Carried at Fair Value and Measured on a Recurring Basis (Detail) - Fair Value Measurements Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 539,612	$ 440,584
US Treasury Securities [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	347,619	252,226
US Treasury Securities [Member] | US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	22,635
Equity Securities [Member] | Other Non-current Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	169,358	188,358
Quoted prices in active markets for identical assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	539,612	440,584
Quoted prices in active markets for identical assets (Level 1) [Member] | US Treasury Securities [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	347,619	252,226
Quoted prices in active markets for identical assets (Level 1) [Member] | US Treasury Securities [Member] | US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	22,635
Quoted prices in active markets for identical assets (Level 1) [Member] | Equity Securities [Member] | Other Non-current Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	169,358	188,358
Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Other Observable Inputs (Level 2) [Member] | US Treasury Securities [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Other Observable Inputs (Level 2) [Member] | US Treasury Securities [Member] | US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Other Observable Inputs (Level 2) [Member] | Equity Securities [Member] | Other Non-current Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Significant Unobservable Inputs (Level 3) [Member] | US Treasury Securities [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Significant Unobservable Inputs (Level 3) [Member] | US Treasury Securities [Member] | US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member] | Other Non-current Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 0	$ 0