Cash, Cash Equivalents, and Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 652,816		$ 396,283
Gross Unrealized Gains	401		107
Gross Unrealized Losses	(128)		(34)
Fair Value	653,089		396,356
Cash and Cash Equivalents	155,443		172,250
Short-Term Investments	406,667		178,534
Long-Term Investments	90,706		45,499
Credit loss reserve for held-to-maturity investments	200	$ 100
Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	183,789		41,714
Gross Unrealized Gains	128		14
Gross Unrealized Losses			(4)
Fair Value	183,917		41,724
Cash and Cash Equivalents	23,611		8,845
Short-Term Investments	120,178		15,131
Long-Term Investments	40,000		17,738
Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	352,920		251,250
Gross Unrealized Gains	273		93
Gross Unrealized Losses	(128)		(30)
Fair Value	353,065		251,313
Cash and Cash Equivalents	15,725		60,086
Short-Term Investments	286,489		163,403
Long-Term Investments	50,706		27,761
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	116,107		103,319
Fair Value	116,107		103,319
Cash and Cash Equivalents	116,107		103,319
Money market funds | Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	23,611		8,845
Fair Value	23,611		8,845
Cash and Cash Equivalents	23,611		8,845
Agency bonds | Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	63,794		32,869
Gross Unrealized Gains	122		14
Gross Unrealized Losses			(4)
Fair Value	63,916		32,879
Short-Term Investments	23,794		15,131
Long-Term Investments	40,000		17,738
Treasury bills | Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	96,384
Gross Unrealized Gains	6
Fair Value	96,390
Short-Term Investments	96,384
State and municipal obligations | Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	77,130		25,038
Gross Unrealized Gains	25		8
Gross Unrealized Losses	(28)
Fair Value	77,127		25,046
Short-Term Investments	66,519		21,560
Long-Term Investments	10,611		3,478
Certificates of deposit | Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	500		1,400
Fair Value	500		1,400
Short-Term Investments	500		1,400
Corporate bonds | Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	212,825		135,175
Gross Unrealized Gains	232		71
Gross Unrealized Losses	(100)		(30)
Fair Value	212,957		135,216
Cash and Cash Equivalents	2,525		886
Short-Term Investments	170,205		113,241
Long-Term Investments	40,095		21,048
U.S. Treasury repurchase agreements | Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	13,200		57,200
Fair Value	13,200		57,200
Cash and Cash Equivalents	13,200		57,200
Treasury inflation - protected securities | Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	3,291		3,235
Gross Unrealized Gains	16		14
Fair Value	3,307		3,249
Short-Term Investments	3,291
Long-Term Investments			3,235
Commercial paper | Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	45,974		29,202
Fair Value	45,974		29,202
Cash and Cash Equivalents			2,000
Short-Term Investments	$ 45,974		$ 27,202