Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Available-for-sale securities:
Available-for-sale securities	$ 1,922.5	$ 1,792.9
Trading securities:
Trading securities	18.9	17.7
Fair Value Measurements (Textuals)
Restricted investments	29.7	29.7
Asset-backed Securities [Member]
Available-for-sale securities:
Available-for-sale securities	324.6	311.7
Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities	18.1	9.6
Commercial paper [Member]
Available-for-sale securities:
Available-for-sale securities	6.7	17.7
Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities	869.6	911.4
Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities	22.8	16.5
Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities	200.1	203.7
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities	129.7	29.7
Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities	6.1	6.2
Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities	8.5	8.8
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities	336.3	277.6
Privately-held debt and redeemable preferred stock securities [Member]
Available-for-sale securities:
Available-for-sale securities	1,778.2	1,748.2
Fair Value, Measurements, Recurring [Member]
Derivative assets:
Total assets measured at fair value	2,013.4	1,871.2
Liabilities measured at fair value:
Other accrued liabilities	(0.6)	(1.3)
Total liabilities measured at fair value	(0.6)	(1.3)
Cash equivalents	134.8	3.4
Restricted investments	35.8	35.9
Short-term investments	599.4	527.1
Long-term investments	1,171.4	1,244.2
Prepaid expenses and other current assets	3.4	0.4
Other long-term assets	68.6	60.2
Fair Value, Measurements, Recurring [Member] | Foreign exchange contract [Member]
Derivative assets:
Foreign exchange contracts	3.4	0.4
Liabilities measured at fair value:
Derivative liabilities	(0.6)	(1.3)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative assets:
Total assets measured at fair value	451.4	309.7
Liabilities measured at fair value:
Other accrued liabilities	0.0	0.0
Total liabilities measured at fair value	0.0	0.0
Cash equivalents	100.0	0.0
Restricted investments	35.8	35.9
Short-term investments	161.2	108.2
Long-term investments	154.4	165.6
Prepaid expenses and other current assets	0.0	0.0
Other long-term assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Foreign exchange contract [Member]
Derivative assets:
Foreign exchange contracts	0.0	0.0
Liabilities measured at fair value:
Derivative liabilities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative assets:
Total assets measured at fair value	1,493.4	1,501.3
Liabilities measured at fair value:
Other accrued liabilities	(0.6)	(1.3)
Total liabilities measured at fair value	(0.6)	(1.3)
Cash equivalents	34.8	3.4
Restricted investments	0.0	0.0
Short-term investments	438.2	418.9
Long-term investments	1,017.0	1,078.6
Prepaid expenses and other current assets	3.4	0.4
Other long-term assets	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign exchange contract [Member]
Derivative assets:
Foreign exchange contracts	3.4	0.4
Liabilities measured at fair value:
Derivative liabilities	(0.6)	(1.3)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative assets:
Total assets measured at fair value	68.6	60.2
Liabilities measured at fair value:
Other accrued liabilities	0.0	0.0
Total liabilities measured at fair value	0.0	0.0
Cash equivalents	0.0	0.0
Restricted investments	0.0	0.0
Short-term investments	0.0	0.0
Long-term investments	0.0	0.0
Prepaid expenses and other current assets	0.0	0.0
Other long-term assets	68.6	60.2
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Foreign exchange contract [Member]
Derivative assets:
Foreign exchange contracts	0.0	0.0
Liabilities measured at fair value:
Derivative liabilities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Available-for-sale securities:
Available-for-sale securities	324.6	311.7
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	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	324.6	311.7
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	0.0
Fair Value, Measurements, Recurring [Member] | Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities	18.1	9.6
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	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	18.1	9.6
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	0.0
Fair Value, Measurements, Recurring [Member] | Commercial paper [Member]
Available-for-sale securities:
Available-for-sale securities	6.7	17.7
Fair Value, Measurements, Recurring [Member] | Commercial paper [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Commercial paper [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	6.7	17.7
Fair Value, Measurements, Recurring [Member] | Commercial paper [Member] | Fair Value, Inputs, Level 3 [Member]
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities	869.6	911.4
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	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	869.6	911.4
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	0.0
Fair Value, Measurements, Recurring [Member] | Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities	22.8	16.5
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	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	22.8	16.5
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	0.0
Fair Value, Measurements, Recurring [Member] | Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities	200.1	203.7
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	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	200.1	203.7
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	0.0
Fair Value, Measurements, Recurring [Member] | Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities	129.7	29.7
Fair Value, Measurements, Recurring [Member] | Money market funds [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	129.7	29.7
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	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	0.0
Fair Value, Measurements, Recurring [Member] | Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities	6.1	6.2
Trading securities:
Trading securities	18.9	17.7
Fair Value, Measurements, Recurring [Member] | Mutual funds [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	6.1	6.2
Trading securities:
Trading securities	18.9	17.7
Fair Value, Measurements, Recurring [Member] | Mutual funds [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Trading securities:
Trading securities	0.0	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	0.0
Trading securities:
Trading securities	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities	8.5	8.8
Fair Value, Measurements, Recurring [Member] | Publicly-traded equity securities [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	8.5	8.8
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	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	0.0
Fair Value, Measurements, Recurring [Member] | US government securities [Member]
Available-for-sale securities:
Available-for-sale securities	336.3	277.6
Fair Value, Measurements, Recurring [Member] | US government securities [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	288.2	247.3
Fair Value, Measurements, Recurring [Member] | US government securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	48.1	30.3
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	0.0
Fair Value, Measurements, Recurring [Member] | Available-for-sale securities [Member]
Available-for-sale securities:
Available-for-sale securities	1,922.5	1,792.9
Fair Value, Measurements, Recurring [Member] | Available-for-sale securities [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	432.5	292.0
Fair Value, Measurements, Recurring [Member] | Available-for-sale securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	1,490.0	1,500.9
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	0.0
Fair Value, Measurements, Recurring [Member] | Privately-held debt and redeemable preferred stock securities [Member]
Trading securities:
Privately-held debt and redeemable preferred stock securities	68.6	60.2
Fair Value, Measurements, Recurring [Member] | Privately-held debt and redeemable preferred stock securities [Member] | Fair Value, Inputs, Level 1 [Member]
Trading securities:
Privately-held debt and redeemable preferred stock securities	0.0	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 debt and redeemable preferred stock securities	0.0	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 debt and redeemable preferred stock securities	68.6	$ 60.2
Fair Value Measurements (Textuals)
Purchases of privately-held investments	$ 10.5