Investments (Details) - USD ($)	6 Months Ended	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Short-term investments	$ 3,300,000		$ 2,300,000
Available-for-sale debt securities	0		0
Sales of available-for-sale investments	$ 0	$ 1,376,600,000	1,380,000,000
Gross realized losses			$ 5,600,000

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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Amount of realized gain (loss) 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 of cash inflow from sale 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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Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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