Cash, Cash Equivalents, and Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 704,117	$ 652,816
Gross Unrealized Gains	195	401
Gross Unrealized Losses	(60)	(128)
Fair Value	704,252	653,089
Cash and Cash Equivalents	266,379	155,443
Short-Term Investments	389,050	406,667
Long-Term Investments	48,688	90,706
Expected credit loss reserve	(200)	(200)
Cash and cash equivalents	266,372	155,440	$ 319,253
Short-term investments	388,895	406,525
Long-term investments	48,669	90,681
Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	218,516	183,789
Gross Unrealized Gains	76	128
Gross Unrealized Losses	(1)
Fair Value	218,591	183,917
Cash and Cash Equivalents	66,152	23,611
Short-Term Investments	144,364	120,178
Long-Term Investments	8,000	40,000
Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	286,175	352,920
Gross Unrealized Gains	119	273
Gross Unrealized Losses	(59)	(128)
Fair Value	286,235	353,065
Cash and Cash Equivalents	801	15,725
Short-Term Investments	244,686	286,489
Long-Term Investments	40,688	50,706
Cash and Cash Equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Expected credit loss reserve	(7)	(3)
Short-Term Investments
Restricted Cash and Cash Equivalents Items [Line Items]
Expected credit loss reserve	(155)	(142)
Long-Term Investments
Restricted Cash and Cash Equivalents Items [Line Items]
Expected credit loss reserve	(19)	(25)
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	199,426	116,107
Fair Value	199,426	116,107
Cash and Cash Equivalents	199,426	116,107
Money market funds | Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	48,052	23,611
Fair Value	48,052	23,611
Cash and Cash Equivalents	48,052	23,611
Agency bonds | Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	73,669	63,794
Gross Unrealized Gains	74	122
Fair Value	73,743	63,916
Cash and Cash Equivalents	5,500
Short-Term Investments	60,169	23,794
Long-Term Investments	8,000	40,000
Treasury bills | Fair Value, Inputs, Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	96,795	96,384
Gross Unrealized Gains	2	6
Gross Unrealized Losses	(1)
Fair Value	96,796	96,390
Cash and Cash Equivalents	12,600
Short-Term Investments	84,195	96,384
State and municipal obligations | Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	101,610	77,130
Gross Unrealized Gains	14	25
Gross Unrealized Losses	(27)	(28)
Fair Value	101,597	77,127
Cash and Cash Equivalents	801
Short-Term Investments	87,596	66,519
Long-Term Investments	13,213	10,611
Certificates of deposit | Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	500	500
Fair Value	500	500
Short-Term Investments	500	500
Corporate bonds | Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	150,082	212,825
Gross Unrealized Gains	105	232
Gross Unrealized Losses	(32)	(100)
Fair Value	150,155	212,957
Cash and Cash Equivalents		2,525
Short-Term Investments	122,607	170,205
Long-Term Investments	27,475	40,095
U.S. Treasury repurchase agreements | Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost		13,200
Fair Value		13,200
Cash and Cash Equivalents		13,200
Treasury inflation - protected securities | Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost		3,291
Gross Unrealized Gains		16
Fair Value		3,307
Short-Term Investments		3,291
Commercial paper | Fair Value, Inputs, Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	33,983	45,974
Fair Value	33,983	45,974
Short-Term Investments	$ 33,983	$ 45,974