Short-term Investments (Available-For-Sale Securities In A Continuous Unrealized Loss Position) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fair Value available-for-sale securities Less than 12 Months	$ 474.2	$ 670.2
Unrealized losses, available-for-sale securities, Less than 12 Months	(0.8)	(1.5)
Fair value available-for-sale securities 12 Months or More	26.6	23.5
Unrealized losses, available-for-sale securities, 12 Months or More	(2.5)	(2.4)
Total Fair Value available-for-sale securities	500.8	693.7
Unrealized losses, available-for-sale securities, Total	(3.3)	(3.9)
U.S. Treasury And Agency Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value available-for-sale securities Less than 12 Months	155.2	256.9
Unrealized losses, available-for-sale securities, Less than 12 Months	(0.2)	(0.4)
Fair value available-for-sale securities 12 Months or More	0.0	0.0
Unrealized losses, available-for-sale securities, 12 Months or More	0.0	0.0
Total Fair Value available-for-sale securities	155.2	256.9
Unrealized losses, available-for-sale securities, Total	(0.2)	(0.4)
Corporate And Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value available-for-sale securities Less than 12 Months	175.6	229.7
Unrealized losses, available-for-sale securities, Less than 12 Months	(0.4)	(0.7)
Fair value available-for-sale securities 12 Months or More	0.0	0.0
Unrealized losses, available-for-sale securities, 12 Months or More	0.0	0.0
Total Fair Value available-for-sale securities	175.6	229.7
Unrealized losses, available-for-sale securities, Total	(0.4)	(0.7)
Certificates of Deposit and Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value available-for-sale securities Less than 12 Months	35.0	3.7
Unrealized losses, available-for-sale securities, Less than 12 Months	0.0	0.0
Fair value available-for-sale securities 12 Months or More	0.0	0.0
Unrealized losses, available-for-sale securities, 12 Months or More	0.0	0.0
Total Fair Value available-for-sale securities	35.0	3.7
Unrealized losses, available-for-sale securities, Total	0.0	0.0
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value available-for-sale securities Less than 12 Months	65.4	151.9
Unrealized losses, available-for-sale securities, Less than 12 Months	(0.1)	(0.3)
Fair value available-for-sale securities 12 Months or More	5.5	2.8
Unrealized losses, available-for-sale securities, 12 Months or More	(0.1)	(0.1)
Total Fair Value available-for-sale securities	70.9	154.7
Unrealized losses, available-for-sale securities, Total	(0.2)	(0.4)
Municipal Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value available-for-sale securities Less than 12 Months	43.0	28.0
Unrealized losses, available-for-sale securities, Less than 12 Months	(0.1)	(0.1)
Fair value available-for-sale securities 12 Months or More	0.0	0.0
Unrealized losses, available-for-sale securities, 12 Months or More	0.0	0.0
Total Fair Value available-for-sale securities	43.0	28.0
Unrealized losses, available-for-sale securities, Total	(0.1)	(0.1)
Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value available-for-sale securities Less than 12 Months	0.0	0.0
Unrealized losses, available-for-sale securities, Less than 12 Months	0.0	0.0
Fair value available-for-sale securities 12 Months or More	21.1	20.7
Unrealized losses, available-for-sale securities, 12 Months or More	(2.4)	(2.3)
Total Fair Value available-for-sale securities	21.1	20.7
Unrealized losses, available-for-sale securities, Total	$ (2.4)	$ (2.3)