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Investments - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Sep. 30, 2017
Mar. 31, 2018
Investments [Abstract]        
Short-term investments $ 4,500,000 $ 4,500,000   $ 1,295,300,000
Available-fore-sale debt securities, adjusted cost       1,292,500,000
Available-for-sale securities, gross realized gains (losses), sale proceeds   $ 1,380,000,000 $ 0  
Available-for-sale securities, recognized losses $ 5,600,000      
Other than temporary impairment losses, available-for-sale securities       $ 15,500,000