Cash, Cash Equivalents and Marketable Securities Available-for-Sale (Schedule of Available-for-Sale Reconciliation) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,211.2	$ 1,249.6
Gross Unrealized Gain	172.5	468.2
Gross Unrealized Loss	(121.9)	(46.2)
Estimated Fair Value	1,261.8	1,671.6
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	129.6	153.0
Gross Unrealized Gain	0.3	0.0
Gross Unrealized Loss	0.0	(0.4)
Estimated Fair Value	129.9	152.6
U.S. government-sponsored agency MBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19.6	29.8
Gross Unrealized Gain	0.1	0.1
Gross Unrealized Loss	(0.1)	(0.4)
Estimated Fair Value	19.6	29.5
Corporate debt - global
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	161.5	219.7
Gross Unrealized Gain	0.6	0.0
Gross Unrealized Loss	(0.2)	(1.6)
Estimated Fair Value	161.9	218.1
Asset backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	25.4	35.6
Gross Unrealized Gain	0.0	0.0
Gross Unrealized Loss	0.0	(0.1)
Estimated Fair Value	25.4	35.5
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	875.1	811.5
Gross Unrealized Gain	171.5	468.1
Gross Unrealized Loss	(121.6)	(43.7)
Estimated Fair Value	$ 925.0	$ 1,235.9