Financial Instruments (Details 2) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, amortized cost	$ 1,413.3	$ 1,278.9
Due in one year or less, estimated fair value	1,413.4	1,278.6
Due after one year through five years, amortized cost	925.5	722.6
Due after one year through five years, estimated fair value	925.3	721.3
Due after five years, amortized cost	42.8	49.5
Due after five years, estimated fair value	42.8	49.2
Estimated Fair Value	2,381.6	2,051.0
Amortized Cost	$ 2,381.5	$ 2,049.1