Short-term Investments (Amortized Cost, Gross Unrealized Gains And Losses And Fair Value Of Investment Securities Available-For-Sale) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,816.8	$ 1,265.1
Unrealized Gains	2.8	1.5
Unrealized Losses	(3.3)	(3.9)
Fair Value	1,816.3	1,262.7
U.S. Treasury And Agency Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	523.6	544.7
Unrealized Gains	1.3	0.4
Unrealized Losses	(0.2)	(0.4)
Fair Value	524.7	544.7
Corporate And Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	373.9	359.0
Unrealized Gains	0.5	0.3
Unrealized Losses	(0.4)	(0.7)
Fair Value	374.0	358.6
Certificates of Deposit and Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	549.4	4.6
Unrealized Gains	0.3	0.0
Unrealized Losses	0.0	0.0
Fair Value	549.7	4.6
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	231.2	220.4
Unrealized Gains	0.2	0.1
Unrealized Losses	(0.2)	(0.4)
Fair Value	231.2	220.1
Municipal Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	114.1	112.5
Unrealized Gains	0.3	0.4
Unrealized Losses	(0.1)	(0.1)
Fair Value	114.3	112.8
Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24.6	23.9
Unrealized Gains	0.2	0.3
Unrealized Losses	(2.4)	(2.3)
Fair Value	$ 22.4	$ 21.9