Fair Value Measurements. Cash Equivalents and Marketable Securities - Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	$ 147,188	$ 140,544
Amortized cost, cash and cash equivalents and debt securities available-for-sale	685,281	381,858
Gross Unrealized Gain	1,288	235
Gross Unrealized Loss	(111)	(317)
Cash, cash equivalents and debt securities, fair value	686,458	381,776
Money market funds
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	8,066	25,796
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated fair value, cash and cash equivalents	8,066	25,796
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale	22,985	42,273
Gross Unrealized Gain	44	0
Gross Unrealized Loss	0	(71)
Estimated fair value, debt securities	23,029	42,202
U.S. government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale	654,230	308,775
Gross Unrealized Gain	1,244	235
Gross Unrealized Loss	(111)	(236)
Estimated fair value, debt securities	$ 655,363	308,774
U.S. government agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale		5,014
Gross Unrealized Gain		0
Gross Unrealized Loss		(10)
Estimated fair value, debt securities		$ 5,004