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Financial Instruments - Schedule Of Available-For-Sale Investments With Unrealized Losses (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Unrealized losses less than 12 months Fair Value $ 1,434.1 $ 1,996.3
Unrealized losses less than 12 months (2.8) (7.2)
Unrealized losses 12 months or greater Fair Value 1,427.6 880.4
Unrealized losses 12 months or greater (11.1) (8.2)
Total Fair Value 2,861.7 2,876.7
Total Unrealized Losses (13.9) (15.4)
Corporate securities    
Schedule of Available-for-sale Securities [Line Items]    
Unrealized losses less than 12 months Fair Value 727.4 567.6
Unrealized losses less than 12 months (1.7) (2.1)
Unrealized losses 12 months or greater Fair Value 409.6 277.0
Unrealized losses 12 months or greater (3.1) (2.3)
Total Fair Value 1,137.0 844.6
Total Unrealized Losses (4.8) (4.4)
U.S. treasuries    
Schedule of Available-for-sale Securities [Line Items]    
Unrealized losses less than 12 months Fair Value 478.7 763.5
Unrealized losses less than 12 months (0.9) (2.5)
Unrealized losses 12 months or greater Fair Value 592.8 206.2
Unrealized losses 12 months or greater (4.4) (2.2)
Total Fair Value 1,071.5 969.7
Total Unrealized Losses (5.3) (4.7)
U.S. government agencies    
Schedule of Available-for-sale Securities [Line Items]    
Unrealized losses less than 12 months Fair Value 228.0 428.9
Unrealized losses less than 12 months (0.2) (1.3)
Unrealized losses 12 months or greater Fair Value 425.2 345.5
Unrealized losses 12 months or greater (3.6) (3.1)
Total Fair Value 653.2 774.4
Total Unrealized Losses $ (3.8) (4.4)
Municipal securities    
Schedule of Available-for-sale Securities [Line Items]    
Unrealized losses less than 12 months Fair Value   236.3
Unrealized losses less than 12 months   (1.3)
Unrealized losses 12 months or greater Fair Value   51.7
Unrealized losses 12 months or greater   (0.6)
Total Fair Value   288.0
Total Unrealized Losses   $ (1.9)