Cash, Cash Equivalents, and Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 686,953		$ 396,283
Gross Unrealized Gains	713		107
Gross Unrealized Losses	(133)		(34)
Fair Value	687,533		396,356
Cash and Cash Equivalents	319,264		172,250
Short-term investments	238,078		178,534
Long-term Investments	129,611		45,499
Investment payable	10,400			$ 0
Credit loss reserve for held-to-maturity investments	100	$ 100
Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	203,831		41,714
Gross Unrealized Gains	98		14
Gross Unrealized Losses	(6)		(4)
Fair Value	203,923		41,724
Cash and Cash Equivalents	100,772		8,845
Short-term investments	39,018		15,131
Long-term Investments	64,041		17,738
Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	341,931		251,250
Gross Unrealized Gains	615		93
Gross Unrealized Losses	(127)		(30)
Fair Value	342,419		251,313
Cash and Cash Equivalents	77,301		60,086
Short-term investments	199,060		163,403
Long-term Investments	65,570		27,761
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	141,191		103,319
Gross Unrealized Losses	0		0
Fair Value	141,191		103,319
Cash and Cash Equivalents	141,191		103,319
Short-term investments	0		0
Money market funds | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	92,773		8,845
Gross Unrealized Losses	0		0
Fair Value	92,773		8,845
Cash and Cash Equivalents	92,773		8,845
Short-term investments	0		0
Agency bonds | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	78,965		32,869
Gross Unrealized Gains	97		14
Gross Unrealized Losses	(6)		(4)
Fair Value	79,056		32,879
Cash and Cash Equivalents	0		0
Short-term investments	14,924		15,131
Long-term Investments	64,041		17,738
Treasury bills | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	32,093
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Fair Value	32,094
Cash and Cash Equivalents	7,999
Short-term investments	24,094
State and municipal obligations | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	55,436		25,038
Gross Unrealized Gains	71		8
Gross Unrealized Losses	(14)		0
Fair Value	55,493		25,046
Cash and Cash Equivalents	6,502		0
Short-term investments	40,892		21,560
Long-term Investments	8,042		3,478
Certificates of Deposit | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	1,900		1,400
Gross Unrealized Losses	0		0
Fair Value	1,900		1,400
Cash and Cash Equivalents	0		0
Short-term investments	1,400		1,400
Long-term Investments	500
Corporate bonds | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	196,247		135,175
Gross Unrealized Gains	515		71
Gross Unrealized Losses	(113)		(30)
Fair Value	196,649		135,216
Cash and Cash Equivalents	9,102		886
Short-term investments	130,117		113,241
Long-term Investments	57,028		21,048
U.S. Treasury repurchase agreements | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	55,500		57,200
Gross Unrealized Losses	0		0
Fair Value	55,500		57,200
Cash and Cash Equivalents	55,500		57,200
Short-term investments	0		0
Treasury inflation - protected securities | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	3,232		3,235
Gross Unrealized Gains	29		14
Gross Unrealized Losses	0		0
Fair Value	3,261		3,249
Cash and Cash Equivalents	0		0
Short-term investments	3,232		0
Long-term Investments			3,235
Commercial Paper | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	29,616		29,202
Gross Unrealized Losses	0		0
Fair Value	29,616		29,202
Cash and Cash Equivalents	6,197		2,000
Short-term investments	$ 23,419		$ 27,202