Investments - Summary of Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Mar. 31, 2018
Summary of available-for-sale securities [Line Items]
Adjusted Cost		$ 1,292,500
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value	$ 0	1,292,500
Government agency bonds
Summary of available-for-sale securities [Line Items]
Adjusted Cost		723,200
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		723,200
Municipal bonds - taxable
Summary of available-for-sale securities [Line Items]
Adjusted Cost		14,900
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		14,900
Time deposits
Summary of available-for-sale securities [Line Items]
Adjusted Cost		11,500
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		11,500
Corporate bonds and debt
Summary of available-for-sale securities [Line Items]
Adjusted Cost		542,900
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		$ 542,900

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).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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