Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available-for-sale securities:
Available-for-sale securities	$ 2,418.3	$ 2,070.8
Trading securities:
Trading securities	17.7	16.3
Fair Value Measurements (Textuals)
Restricted investments	41.2	41.3
Asset-backed Securities [Member]
Available-for-sale securities:
Available-for-sale securities	308.8	269.0
Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities	8.2	10.6
Commercial paper [Member]
Available-for-sale securities:
Available-for-sale securities	15.6	20.3
Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities	843.8	738.0
Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities	34.7	24.6
Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities	190.4	162.1
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities	776.2	594.2
Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities	4.1	4.0
Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities	2.6	2.0
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities	233.9	246.0
Privately-held debt and redeemable preferred stock securities [Member]
Available-for-sale securities:
Available-for-sale securities	1,635.4	1,470.6
Fair Value, Measurements, Recurring [Member]
Derivative assets:
Total assets measured at fair value	2,486.8	2,134.7
Liabilities measured at fair value:
Other accrued liabilities	(6.7)	(3.9)
Total liabilities measured at fair value	(6.7)	(3.9)
Cash equivalents	750.8	576.6
Restricted investments	45.4	45.2
Short-term investments	398.2	332.2
Long-term investments	1,241.6	1,133.1
Prepaid expenses and other current assets	0.5	0.1
Other long-term assets	50.3	47.5
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative assets:
Total assets measured at fair value	1,022.2	862.5
Liabilities measured at fair value:
Other accrued liabilities	0	0
Total liabilities measured at fair value	0	0
Cash equivalents	735.0	552.9
Restricted investments	45.4	45.2
Short-term investments	65.2	87.0
Long-term investments	176.6	177.4
Prepaid expenses and other current assets	0	0
Other long-term assets	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative assets:
Total assets measured at fair value	1,414.3	1,224.7
Liabilities measured at fair value:
Other accrued liabilities	(6.7)	(3.9)
Total liabilities measured at fair value	(6.7)	(3.9)
Cash equivalents	15.8	23.7
Restricted investments	0	0
Short-term investments	333.0	245.2
Long-term investments	1,065.0	955.7
Prepaid expenses and other current assets	0.5	0.1
Other long-term assets	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative assets:
Total assets measured at fair value	50.3	47.5
Liabilities measured at fair value:
Other accrued liabilities	0	0
Total liabilities measured at fair value	0	0
Cash equivalents	0	0
Restricted investments	0	0
Short-term investments	0	0
Long-term investments	0	0
Prepaid expenses and other current assets	0	0
Other long-term assets	50.3	47.5
Fair Value, Measurements, Recurring [Member] | Foreign exchange contract [Member]
Derivative assets:
Derivative assets	0.5	0.1
Liabilities measured at fair value:
Derivative liabilities	(6.7)	(3.9)
Fair Value, Measurements, Recurring [Member] | Foreign exchange contract [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative assets:
Derivative assets	0	0
Liabilities measured at fair value:
Derivative liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Foreign exchange contract [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative assets:
Derivative assets	0.5	0.1
Liabilities measured at fair value:
Derivative liabilities	(6.7)	(3.9)
Fair Value, Measurements, Recurring [Member] | Foreign exchange contract [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative assets:
Derivative assets	0	0
Liabilities measured at fair value:
Derivative liabilities	0	0
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Available-for-sale securities:
Available-for-sale securities	308.8	269.0
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	308.8	269.0
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities	8.2	10.6
Fair Value, Measurements, Recurring [Member] | Certificates of deposit [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Certificates of deposit [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	8.2	10.6
Fair Value, Measurements, Recurring [Member] | Certificates of deposit [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Commercial paper [Member]
Available-for-sale securities:
Available-for-sale securities	15.6	20.3
Fair Value, Measurements, Recurring [Member] | Commercial paper [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Commercial paper [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	15.6	20.3
Fair Value, Measurements, Recurring [Member] | Commercial paper [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities	843.8	738.0
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	843.8	738.0
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities	34.7	24.6
Fair Value, Measurements, Recurring [Member] | Foreign government debt securities [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Foreign government debt securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	34.7	24.6
Fair Value, Measurements, Recurring [Member] | Foreign government debt securities [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities	190.4	162.1
Fair Value, Measurements, Recurring [Member] | Government-sponsored enterprise obligations [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Government-sponsored enterprise obligations [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	190.4	162.1
Fair Value, Measurements, Recurring [Member] | Government-sponsored enterprise obligations [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities	776.2	594.2
Fair Value, Measurements, Recurring [Member] | Money market funds [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	776.2	594.2
Fair Value, Measurements, Recurring [Member] | Money market funds [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Money market funds [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities	4.1	4.0
Trading securities:
Trading securities	17.7	16.3
Fair Value, Measurements, Recurring [Member] | Mutual funds [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	4.1	4.0
Trading securities:
Trading securities	17.7	16.3
Fair Value, Measurements, Recurring [Member] | Mutual funds [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Trading securities:
Trading securities	0	0
Fair Value, Measurements, Recurring [Member] | Mutual funds [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Trading securities:
Trading securities	0	0
Fair Value, Measurements, Recurring [Member] | Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities	2.6	2.0
Fair Value, Measurements, Recurring [Member] | Publicly-traded equity securities [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	2.6	2.0
Fair Value, Measurements, Recurring [Member] | Publicly-traded equity securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Publicly-traded equity securities [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | US government securities [Member]
Available-for-sale securities:
Available-for-sale securities	233.9	246.0
Fair Value, Measurements, Recurring [Member] | US government securities [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	221.6	246.0
Fair Value, Measurements, Recurring [Member] | US government securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	12.3	0
Fair Value, Measurements, Recurring [Member] | US government securities [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Available-for-sale securities [Member]
Available-for-sale securities:
Available-for-sale securities	2,418.3	2,070.8
Fair Value, Measurements, Recurring [Member] | Available-for-sale securities [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	1,004.5	846.2
Fair Value, Measurements, Recurring [Member] | Available-for-sale securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	1,413.8	1,224.6
Fair Value, Measurements, Recurring [Member] | Available-for-sale securities [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Privately-held debt and redeemable preferred stock securities [Member]
Trading securities:
Privately-held securities	50.3	47.5
Fair Value, Measurements, Recurring [Member] | Privately-held debt and redeemable preferred stock securities [Member] | Fair Value, Inputs, Level 1 [Member]
Trading securities:
Privately-held securities	0	0
Fair Value, Measurements, Recurring [Member] | Privately-held debt and redeemable preferred stock securities [Member] | Fair Value, Inputs, Level 2 [Member]
Trading securities:
Privately-held securities	0	0
Fair Value, Measurements, Recurring [Member] | Privately-held debt and redeemable preferred stock securities [Member] | Fair Value, Inputs, Level 3 [Member]
Trading securities:
Privately-held securities	$ 50.3	$ 47.5