Fair Value Measurements, Cash Equivalents and Marketable Securities - Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	$ 164,671	$ 143,228
Amortized cost, cash and cash equivalents and debt securities available-for-sale	933,892	657,696
Gross Unrealized Gain	5,623	1,434
Gross Unrealized Loss	0	(39)
Cash, cash equivalents and debt securities, fair value	939,515	659,091
Money market funds
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	20,237	10,734
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated fair value, cash and cash equivalents	20,237	10,734
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale		16,679
Gross Unrealized Gain		11
Gross Unrealized Loss		0
Estimated fair value, debt securities		16,690
U.S. government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale	913,655	630,283
Gross Unrealized Gain	5,623	1,423
Gross Unrealized Loss	0	(39)
Estimated fair value, debt securities	$ 919,278	$ 631,667