Investments - Summary of Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	$ 1,694,942	$ 1,517,783
Available-for-sale securities, gross unrealized gains	3,148	367
Available-for-sale securities, gross unrealized losses	(1,066)	(4,576)
Available-for-sale securities, estimated fair value	1,697,024	1,513,574
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	137,523	108,061
Available-for-sale securities, gross unrealized gains	101	0
Available-for-sale securities, gross unrealized losses	(6)	0
Available-for-sale securities, estimated fair value	137,618	108,061
Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	1,405,920	1,233,589
Available-for-sale securities, gross unrealized gains	2,955	343
Available-for-sale securities, gross unrealized losses	(903)	(4,218)
Available-for-sale securities, estimated fair value	1,407,972	1,229,714
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	60,337	73,584
Available-for-sale securities, gross unrealized gains	43	1
Available-for-sale securities, gross unrealized losses	0	0
Available-for-sale securities, estimated fair value	60,380	73,585
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	91,162	102,549
Available-for-sale securities, gross unrealized gains	49	23
Available-for-sale securities, gross unrealized losses	(157)	(358)
Available-for-sale securities, estimated fair value	$ 91,054	$ 102,214

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Name Accounting Standards Codification
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Paragraph 1
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