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Financial Instruments (Details 2) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Summary of Contractual Maturities: Available-for-Sale Securities    
Due in one year or less, estimated fair value $ 2,041.7 $ 2,115.2
Due in one year or less, amortized cost 2,042.5 2,116.0
Due after one year through five years, estimated fair value 1,111.2 2,730.0
Due after one year through five years, amortized cost 1,112.3 2,730.0
Due after five years, estimated fair value 133.3 327.3
Due after five years, amortized cost 133.6 327.0
Fair value 3,286.2 5,172.5
Amortized cost $ 3,288.4 $ 5,173.0