Fair Value Measurements - Summary of Placement in the Fair Value Hierarchy of Financial Assets that are Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets
Fair value of financial assets measured at fair value on a recurring basis	$ 89,519	$ 122,181
Quoted Prices in Active Markets (Level 1)
Assets
Fair value of financial assets measured at fair value on a recurring basis	1,196	1,626
Significant Other Observable Inputs (Level 2)
Assets
Fair value of financial assets measured at fair value on a recurring basis	68,035	120,364
Significant Unobservable Inputs (Level 3)
Assets
Fair value of financial assets measured at fair value on a recurring basis	20,288	191
U.S. government debt securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	67,301	119,340
U.S. government debt securities | Quoted Prices in Active Markets (Level 1)
Assets
Fair value of financial assets measured at fair value on a recurring basis	0	0
U.S. government debt securities | Significant Other Observable Inputs (Level 2)
Assets
Fair value of financial assets measured at fair value on a recurring basis	67,301	119,340
U.S. government debt securities | Significant Unobservable Inputs (Level 3)
Assets
Fair value of financial assets measured at fair value on a recurring basis	0	0
Equity securities
Assets
Fair value of financial assets measured at fair value on a recurring basis	21,503	2,182
Equity securities | Quoted Prices in Active Markets (Level 1)
Assets
Fair value of financial assets measured at fair value on a recurring basis	1,196	1,626
Equity securities | Significant Other Observable Inputs (Level 2)
Assets
Fair value of financial assets measured at fair value on a recurring basis	266	556
Equity securities | Significant Unobservable Inputs (Level 3)
Assets
Fair value of financial assets measured at fair value on a recurring basis	20,041	0
Other
Assets
Fair value of financial assets measured at fair value on a recurring basis	715	659
Other | Quoted Prices in Active Markets (Level 1)
Assets
Fair value of financial assets measured at fair value on a recurring basis	0	0
Other | Significant Other Observable Inputs (Level 2)
Assets
Fair value of financial assets measured at fair value on a recurring basis	468	468
Other | Significant Unobservable Inputs (Level 3)
Assets
Fair value of financial assets measured at fair value on a recurring basis	$ 247	$ 191