Financial Instruments (Details 2) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, amortized cost	$ 1,942.7	$ 1,562.2
Due in one year or less, estimated fair value	1,940.8	1,560.8
Due after one year through five years, amortized cost	862.6	1,234.5
Due after one year through five years, estimated fair value	858.5	1,230.4
Due after five years, amortized cost	60.2	173.6
Due after five years, estimated fair value	59.8	173.8
Amortized cost	2,859.1	2,965.0
Available-for-sale Securities	$ 2,865.5	$ 2,970.3