Financial Instruments (Details 2) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, amortized cost	$ 1,308.8	$ 1,278.9
Due in one year or less, estimated fair value	1,308.9	1,278.6
Due after one year through five years, amortized cost	870.9	722.6
Due after one year through five years, estimated fair value	870.5	721.3
Due after five years, amortized cost	44.2	49.5
Due after five years, estimated fair value	44.2	49.2
Amortized cost	2,223.6	2,049.1
Available-for-sale Securities	$ 2,223.9	$ 2,051.0