Investments - AFS (Details) - USD ($) $ in Millions	Dec. 31, 2018	Mar. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost		$ 1,292.5
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Estimated Fair Value		1,292.5
Government agency bonds
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost		723.2
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Estimated Fair Value		723.2
Municipal bonds - taxable
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost		14.9
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Estimated Fair Value		14.9
Time deposits
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost	$ 2.3	11.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	$ 2.3	11.5
Corporate bonds and debt
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost		542.9
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Estimated Fair Value		$ 542.9

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

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 (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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