Fair Value - Summary of Debt Securities (Details) - US Treasury Securities [Member] - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 798,269	$ 593,565
Gross unrealized gains	670	236
Gross unrealized losses	(18)	(25)
Fair value	798,921	593,776
Debt securities, due in one year or less, amortized cost	670,578	466,439
Debt securities, due in one year or less, fair value	670,663	466,547
Debt securities, due in one to two years, amortized cost	127,691	127,126
Debt securities, due in one to two years, fair value	$ 128,258	$ 127,229

Debt Securities, Available-For-Sale, Maturity, After One Through Two Years, Amortized Cost

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Debt Securities, Available-For-Sale, Maturity, After One Through Two Years, Fair Value

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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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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).

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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).

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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), with single maturity date and allocated without single maturity date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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