Cash, Cash Equivalents and Marketable Securities Available-for-Sale (Schedule of Available-for-Sale Reconciliation) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,230.8	$ 1,249.6
Gross Unrealized Gain	215.3	468.2
Gross Unrealized Loss	(106.1)	(46.2)
Estimated Fair Value	1,340.0	1,671.6
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	124.6	153.0
Gross Unrealized Gain	0.4	0.0
Gross Unrealized Loss	0.0	(0.4)
Estimated Fair Value	125.0	152.6
U.S. government-sponsored agency MBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28.6	29.8
Gross Unrealized Gain	0.1	0.1
Gross Unrealized Loss	0.0	(0.4)
Estimated Fair Value	28.7	29.5
Corporate debt - global
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	163.9	219.7
Gross Unrealized Gain	1.1	0.0
Gross Unrealized Loss	(0.1)	(1.6)
Estimated Fair Value	164.9	218.1
Asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	26.5	35.6
Gross Unrealized Gain	0.1	0.0
Gross Unrealized Loss	0.0	(0.1)
Estimated Fair Value	26.6	35.5
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	887.2	811.5
Gross Unrealized Gain	213.6	468.1
Gross Unrealized Loss	(106.0)	(43.7)
Estimated Fair Value	$ 994.8	$ 1,235.9