Short-term Investments (Available-For-Sale Securities In A Continuous Unrealized Loss Position) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Fair Value available-for-sale securities Less than 12 Months	$ 269.4	$ 1,318.2
Unrealized losses, available-for-sale securities, Less than 12 Months	(0.5)	(4.2)
Fair value available-for-sale securities 12 Months or More	35.1	35.6
Unrealized losses, available-for-sale securities, 12 Months or More	(3.0)	(3.2)
Total Fair Value available-for-sale securities	304.5	1,353.8
Unrealized losses, available-for-sale securities, Total	(3.5)	(7.4)
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	43.0	475.7
Unrealized losses, available-for-sale securities, Less than 12 Months	0.0	(1.8)
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	475.7
Unrealized losses, available-for-sale securities, Total	0.0	(1.8)
Corporate And Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value available-for-sale securities Less than 12 Months	99.0	315.1
Unrealized losses, available-for-sale securities, Less than 12 Months	(0.2)	(1.5)
Fair value available-for-sale securities 12 Months or More	3.1	3.1
Unrealized losses, available-for-sale securities, 12 Months or More	0.0	0.0
Total Fair Value available-for-sale securities	102.1	318.2
Unrealized losses, available-for-sale securities, Total	(0.2)	(1.5)
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		271.5
Unrealized losses, available-for-sale securities, Less than 12 Months		(0.1)
Fair value available-for-sale securities 12 Months or More		0.0
Unrealized losses, available-for-sale securities, 12 Months or More		0.0
Total Fair Value available-for-sale securities		271.5
Unrealized losses, available-for-sale securities, Total		(0.1)
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value available-for-sale securities Less than 12 Months	103.9	199.4
Unrealized losses, available-for-sale securities, Less than 12 Months	(0.1)	(0.7)
Fair value available-for-sale securities 12 Months or More	10.5	11.4
Unrealized losses, available-for-sale securities, 12 Months or More	(0.1)	(0.1)
Total Fair Value available-for-sale securities	114.4	210.8
Unrealized losses, available-for-sale securities, Total	(0.2)	(0.8)
Municipal Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value available-for-sale securities Less than 12 Months	22.4	56.5
Unrealized losses, available-for-sale securities, Less than 12 Months	0.0	(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	22.4	56.5
Unrealized losses, available-for-sale securities, Total	0.0	(0.1)
Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value available-for-sale securities Less than 12 Months	1.1	0.0
Unrealized losses, available-for-sale securities, Less than 12 Months	(0.2)	0.0
Fair value available-for-sale securities 12 Months or More	21.5	21.1
Unrealized losses, available-for-sale securities, 12 Months or More	(2.9)	(3.1)
Total Fair Value available-for-sale securities	22.6	21.1
Unrealized losses, available-for-sale securities, Total	$ (3.1)	$ (3.1)