Marketable Investment Securities - Narrative (Details) $ in Millions |
Sep. 30, 2020
USD ($)
|
|---|---|
| Investments, Debt and Equity Securities [Abstract] | |
| Available-for-sale securities with contractual maturities of one year of less | $ 1,900.0 |
| Available-for-sale debt securities with contractual maturities greater than one year | $ 7.9 |