Cash, Cash Equivalents and Marketable Securities Available-for-Sale (Schedule of Available-for-Sale Reconcilation) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 2,310.5 $ 2,712.1
Gross Unrealized Gain 648.2 718.3
Gross Unrealized Loss (12.9) (5.3)
Estimated Fair Value 2,945.8 3,425.1
U.S. Treasury securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 587.3 1,044.7
Gross Unrealized Gain 1.2 0.3
Gross Unrealized Loss (0.1) (0.8)
Estimated Fair Value 588.4 1,044.2
U.S. government-sponsored agency securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 97.0 145.1
Gross Unrealized Gain 0.3 0.1
Gross Unrealized Loss 0 (0.1)
Estimated Fair Value 97.3 145.1
U.S. government-sponsored agency MBS    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 357.6 531.1
Gross Unrealized Gain 1.3 1.0
Gross Unrealized Loss (1.4) (2.7)
Estimated Fair Value 357.5 529.4
Non-U.S. government, agency and Supranational securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 27.8 32.4
Gross Unrealized Gain 0 0
Gross Unrealized Loss 0 (0.1)
Estimated Fair Value 27.8 32.3
Corporate debt - global    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 761.6 446.3
Gross Unrealized Gain 2.4 0.6
Gross Unrealized Loss (0.3) (1.2)
Estimated Fair Value 763.7 445.7
Asset backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 144.0 177.3
Gross Unrealized Gain 0.1 0
Gross Unrealized Loss (0.1) (0.2)
Estimated Fair Value 144.0 177.1
Marketable equity securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 335.2 335.2
Gross Unrealized Gain 642.9 716.3
Gross Unrealized Loss (11.0) (0.2)
Estimated Fair Value $ 967.1 $ 1,051.3