Cash, Cash Equivalents, and Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 627,698		$ 396,283
Gross Unrealized Gains	606		107
Gross Unrealized Losses	(180)		(34)
Fair Value	628,124		396,356
Cash and Cash Equivalents	176,005		172,250
Short-term investments	331,040		178,534
Long-term Investments	120,653		45,499
Credit loss reserve for held-to-maturity investments	200	$ 100
Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	156,693		41,714
Gross Unrealized Gains	144		14
Gross Unrealized Losses	0		(4)
Fair Value	156,837		41,724
Cash and Cash Equivalents	36,797		8,845
Short-term investments	76,395		15,131
Long-term Investments	43,501		17,738
Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	379,506		251,250
Gross Unrealized Gains	462		93
Gross Unrealized Losses	(180)		(30)
Fair Value	379,788		251,313
Cash and Cash Equivalents	47,709		60,086
Short-term investments	254,645		163,403
Long-term Investments	77,152		27,761
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	91,499		103,319
Gross Unrealized Losses	0		0
Fair Value	91,499		103,319
Cash and Cash Equivalents	91,499		103,319
Short-term investments	0		0
Money market funds | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	31,298		8,845
Gross Unrealized Losses	0		0
Fair Value	31,298		8,845
Cash and Cash Equivalents	31,298		8,845
Short-term investments	0		0
Agency bonds | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	68,111		32,869
Gross Unrealized Gains	141		14
Gross Unrealized Losses	0		(4)
Fair Value	68,252		32,879
Cash and Cash Equivalents	0		0
Short-term investments	24,610		15,131
Long-term Investments	43,501		17,738
Treasury bills | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	57,284
Gross Unrealized Gains	3
Gross Unrealized Losses	0
Fair Value	57,287
Cash and Cash Equivalents	5,499
Short-term investments	51,785
State and municipal obligations | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	63,363		25,038
Gross Unrealized Gains	49		8
Gross Unrealized Losses	(14)		0
Fair Value	63,398		25,046
Cash and Cash Equivalents	1,001		0
Short-term investments	49,441		21,560
Long-term Investments	12,921		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	222,817		135,175
Gross Unrealized Gains	389		71
Gross Unrealized Losses	(166)		(30)
Fair Value	223,040		135,216
Cash and Cash Equivalents	2,908		886
Short-term investments	156,178		113,241
Long-term Investments	63,731		21,048
U.S. Treasury repurchase agreements | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	43,800		57,200
Gross Unrealized Losses	0		0
Fair Value	43,800		57,200
Cash and Cash Equivalents	43,800		57,200
Short-term investments	0		0
Treasury inflation - protected securities | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	3,270		3,235
Gross Unrealized Gains	24		14
Gross Unrealized Losses	0		0
Fair Value	3,294		3,249
Cash and Cash Equivalents	0		0
Short-term investments	3,270		0
Long-term Investments			3,235
Commercial Paper | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	44,356		29,202
Gross Unrealized Losses	0		0
Fair Value	44,356		29,202
Cash and Cash Equivalents	0		2,000
Short-term investments	$ 44,356		$ 27,202