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FINANCIAL INSTRUMENTS - Summary of Cash and Available-For-Sale Securities (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]    
Cash and cash equivalents $ 875.7 $ 857.9
Total 4,199.0  
Total assets measured at fair value, Amortized Cost 5,059.7 4,845.0
Gross Unrealized Gains 11.7 3.3
Gross Unrealized Losses (6.8) (13.9)
Total assets measured at fair value, Fair Value 5,064.6 4,834.4
Short- term Investments 1,933.0 2,205.2
Long- term Investments 2,255.9 1,771.3
Cash    
Schedule of Available-for-sale Securities [Line Items]    
Cash and cash equivalents 251.5 269.4
Fair Value 251.5 269.4
Level 1    
Schedule of Available-for-sale Securities [Line Items]    
Cash and cash equivalents 621.2 579.1
Total 2,191.4 2,046.9
Gross Unrealized Gains 3.7 1.7
Gross Unrealized Losses (3.0) (5.3)
Fair Value 2,192.1 2,043.3
Short- term Investments 794.0 897.8
Long- term Investments 776.9 566.4
Level 1 | Money market funds    
Schedule of Available-for-sale Securities [Line Items]    
Cash and cash equivalents 609.2 569.1
Fair Value 609.2 569.1
Short- term Investments 0.0 0.0
Long- term Investments 0.0 0.0
Level 1 | U.S. treasuries    
Schedule of Available-for-sale Securities [Line Items]    
Cash and cash equivalents 12.0 10.0
Total 1,582.2 1,477.8
Gross Unrealized Gains 3.7 1.7
Gross Unrealized Losses (3.0) (5.3)
Fair Value 1,582.9 1,474.2
Short- term Investments 794.0 897.8
Long- term Investments 776.9 566.4
Level 2    
Schedule of Available-for-sale Securities [Line Items]    
Cash and cash equivalents 3.0 9.4
Total 2,616.8 2,528.7
Gross Unrealized Gains 8.0 1.6
Gross Unrealized Losses (3.8) (8.6)
Fair Value 2,621.0 2,521.7
Short- term Investments 1,139.0 1,307.4
Long- term Investments 1,479.0 1,204.9
Level 2 | Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Cash and cash equivalents 3.0 1.4
Total 97.3 110.7
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Fair Value 97.3 110.7
Short- term Investments 94.3 109.3
Long- term Investments 0.0 0.0
Level 2 | Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Cash and cash equivalents 0.0 8.0
Total 1,786.5 1,607.8
Gross Unrealized Gains 7.3 1.3
Gross Unrealized Losses (1.7) (4.8)
Fair Value 1,792.1 1,604.3
Short- term Investments 679.7 724.5
Long- term Investments 1,112.4 871.8
Level 2 | U.S. government agencies    
Schedule of Available-for-sale Securities [Line Items]    
Cash and cash equivalents 0.0 0.0
Total 709.1 791.8
Gross Unrealized Gains 0.6 0.3
Gross Unrealized Losses (2.1) (3.8)
Fair Value 707.6 788.3
Short- term Investments 360.3 468.9
Long- term Investments 347.3 319.4
Level 2 | Municipal securities    
Schedule of Available-for-sale Securities [Line Items]    
Cash and cash equivalents 0.0 0.0
Total 23.9 18.4
Gross Unrealized Gains 0.1 0.0
Gross Unrealized Losses 0.0 0.0
Fair Value 24.0 18.4
Short- term Investments 4.7 4.7
Long- term Investments $ 19.3 13.7
Prepaids and other current assets    
Schedule of Available-for-sale Securities [Line Items]    
Restricted cash equivalents   $ 36.5