Cash, Cash Equivalents and Marketable Securities Available-for-Sale (Continuous Unrealized Loss Positions) (Details) $ in Millions	Dec. 31, 2015 USD ($)
Estimated Fair Value
Less than 12 months	$ 873.9
12 months or longer	12.7
Total	886.6
Gross Unrealized Loss
Less than 12 months	(46.1)
12 months or longer	(0.1)
Total	(46.2)
U.S. Treasury securities
Estimated Fair Value
Less than 12 months	143.6
12 months or longer	0.0
Total	143.6
Gross Unrealized Loss
Less than 12 months	(0.4)
12 months or longer	0.0
Total	(0.4)
U.S. government-sponsored agency MBS
Estimated Fair Value
Less than 12 months	12.8
12 months or longer	6.8
Total	19.6
Gross Unrealized Loss
Less than 12 months	(0.3)
12 months or longer	(0.1)
Total	(0.4)
Corporate debt – global
Estimated Fair Value
Less than 12 months	193.5
12 months or longer	0.0
Total	193.5
Gross Unrealized Loss
Less than 12 months	(1.6)
12 months or longer	0.0
Total	(1.6)
Asset backed securities
Estimated Fair Value
Less than 12 months	29.6
12 months or longer	5.9
Total	35.5
Gross Unrealized Loss
Less than 12 months	(0.1)
12 months or longer	0.0
Total	(0.1)
Marketable equity securities
Estimated Fair Value
Less than 12 months	494.4
12 months or longer	0.0
Total	494.4
Gross Unrealized Loss
Less than 12 months	(43.7)
12 months or longer	0.0
Total	$ (43.7)