Financial Instruments (Details 2) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, amortized cost	$ 1,320.0	$ 1,278.9
Due in one year or less, estimated fair value	1,320.2	1,278.6
Due after one year through five years, amortized cost	781.2	722.6
Due after one year through five years, estimated fair value	780.4	721.3
Due after five years, amortized cost	40.7	49.5
Due after five years, estimated fair value	40.3	49.2
Amortized cost	2,140.9	2,049.1
Available-for-sale Securities	$ 2,141.9	$ 2,051.0