Investments - Narrative (Details) - USD ($)	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Investments [Abstract]
Marketable equity securities	$ 2,300,000	$ 2,800,000
Short-term investments	2,300,000	1,295,300,000
Sales of available-for-sale investments	1,376,600,000	0	$ 470,200,000
Gross realized losses	5,600,000
Realized gain on available-for-sale investments		0	$ 0
Impairment of available-for-sale debt securities		15,500,000
Available-for-sale debt securities, unrealized loss	0	0
Estimated Fair Value	$ 0	$ 1,292,500,000