Cash, Cash Equivalents, and Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	$ 395,537	$ 396,283
Gross Unrealized Gains	219	107
Gross Unrealized Losses	(1,035)	(34)
Fair Value	394,721	396,356
Cash and Cash Equivalents	156,541	172,250
Short-term investments	188,759	178,534
Long-term Investments	50,237	45,499
Investment payable	13,451		$ 0
Credit loss reserve for held-to-maturity investments	100	100
Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	61,859	41,714
Gross Unrealized Gains	106	14
Gross Unrealized Losses	(2)	(4)
Fair Value	61,963	41,724
Cash and Cash Equivalents	16,531	8,845
Short-term investments	19,726	15,131
Long-term Investments	25,602	17,738
Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	260,118	251,250
Gross Unrealized Gains	113	93
Gross Unrealized Losses	(1,033)	(30)
Fair Value	259,198	251,313
Cash and Cash Equivalents	66,450	60,086
Short-term investments	169,033	163,403
Long-term Investments	24,635	27,761
Cash
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	73,560	103,319
Gross Unrealized Losses	0	0
Fair Value	73,560	103,319
Cash and Cash Equivalents	73,560	103,319
Short-term investments	0	0
Money market funds | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	3,536	8,845
Gross Unrealized Losses	0	0
Fair Value	3,536	8,845
Cash and Cash Equivalents	3,536	8,845
Short-term investments	0	0
Agency bonds | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	40,340	32,869
Gross Unrealized Gains	91	14
Gross Unrealized Losses	(2)	(4)
Fair Value	40,429	32,879
Cash and Cash Equivalents	2,000	0
Short-term investments	12,738	15,131
Long-term Investments	25,602	17,738
Treasury bills | Level 1
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	17,983
Gross Unrealized Gains	15
Fair Value	17,998
Cash and Cash Equivalents	10,995
Short-term investments	6,988
State and municipal obligations | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	32,185	25,038
Gross Unrealized Gains	34	8
Gross Unrealized Losses	(27)	0
Fair Value	32,192	25,046
Cash and Cash Equivalents	1,991	0
Short-term investments	29,687	21,560
Long-term Investments	507	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	145,022	135,175
Gross Unrealized Gains	79	71
Gross Unrealized Losses	(959)	(30)
Fair Value	144,142	135,216
Cash and Cash Equivalents	4,987	886
Short-term investments	119,654	113,241
Long-term Investments	20,381	21,048
U.S. Treasury repurchase agreements | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	49,400	57,200
Gross Unrealized Losses	0	0
Fair Value	49,400	57,200
Cash and Cash Equivalents	49,400	57,200
Short-term investments	0	0
Treasury inflation - protected securities | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	3,247	3,235
Gross Unrealized Gains		14
Gross Unrealized Losses	(47)	0
Fair Value	3,200	3,249
Cash and Cash Equivalents	0	0
Short-term investments	0	0
Long-term Investments	3,247	3,235
Commercial Paper | Level 2
Restricted Cash and Cash Equivalents Items [Line Items]
Amortized Cost	28,364	29,202
Gross Unrealized Losses	0	0
Fair Value	28,364	29,202
Cash and Cash Equivalents	10,072	2,000
Short-term investments	$ 18,292	$ 27,202