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Financial Instruments (Unrealized Gains and Losses) (Details) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Fair Value Disclosures [Abstract]    
Unrealized gross gains $ 9.3 $ 0.8
Unrealized gross losses 4.1 29.0
Fair value of securities in an unrealized gain position 441.9 84.3
Fair value of securities in an unrealized loss position $ 255.8 $ 858.6