Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale securities:
Available-for-sale securities	$ 2,221.2	$ 2,221.2	$ 1,792.9
Fair Value Measurements (Textuals)
Restricted investments	43.0	43.0	29.7
Asset-backed Securities [Member]
Available-for-sale securities:
Available-for-sale securities	327.4	327.4	311.7
Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities	49.9	49.9	9.6
Commercial paper [Member]
Available-for-sale securities:
Available-for-sale securities	52.4	52.4	17.7
Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities	867.3	867.3	911.4
Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities	38.5	38.5	16.5
Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities	127.4	127.4	203.7
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities	413.0	413.0	29.7
Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities	8.1	8.1	6.2
Trading securities:
Trading securities	20.5	20.5	17.7
Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities	5.3	5.3	8.8
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities	331.9	331.9	277.6
Privately-held debt and redeemable preferred stock securities [Member]
Available-for-sale securities:
Available-for-sale securities	1,794.8	1,794.8	1,748.2
Fair Value, Measurements, Recurring [Member]
Derivative assets:
Total assets measured at fair value	2,288.1	2,288.1	1,871.2
Liabilities measured at fair value:
Other accrued liabilities	(0.9)	(0.9)	(1.3)
Total liabilities measured at fair value	(0.9)	(0.9)	(1.3)
Cash equivalents	379.0	379.0	3.4
Restricted investments	71.6	71.6	35.9
Short-term investments	632.7	632.7	527.1
Long-term investments	1,158.4	1,158.4	1,244.2
Prepaid expenses and other current assets	2.7	2.7	0.4
Other long-term assets	43.7	43.7	60.2
Fair Value, Measurements, Recurring [Member] | Foreign exchange contract [Member]
Derivative assets:
Foreign exchange contracts	2.7	2.7	0.4
Liabilities measured at fair value:
Derivative liabilities	(0.9)	(0.9)	(1.3)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative assets:
Total assets measured at fair value	767.8	767.8	309.7
Liabilities measured at fair value:
Other accrued liabilities	0.0	0.0	0.0
Total liabilities measured at fair value	0.0	0.0	0.0
Cash equivalents	370.0	370.0	0.0
Restricted investments	71.6	71.6	35.9
Short-term investments	173.1	173.1	108.2
Long-term investments	153.1	153.1	165.6
Prepaid expenses and other current assets	0.0	0.0	0.0
Other long-term assets	0.0	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	0.0
Liabilities measured at fair value:
Derivative liabilities	0.0	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative assets:
Total assets measured at fair value	1,476.6	1,476.6	1,501.3
Liabilities measured at fair value:
Other accrued liabilities	(0.9)	(0.9)	(1.3)
Total liabilities measured at fair value	(0.9)	(0.9)	(1.3)
Cash equivalents	9.0	9.0	3.4
Restricted investments	0.0	0.0	0.0
Short-term investments	459.6	459.6	418.9
Long-term investments	1,005.3	1,005.3	1,078.6
Prepaid expenses and other current assets	2.7	2.7	0.4
Other long-term assets	0.0	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign exchange contract [Member]
Derivative assets:
Foreign exchange contracts	2.7	2.7	0.4
Liabilities measured at fair value:
Derivative liabilities	(0.9)	(0.9)	(1.3)
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative assets:
Total assets measured at fair value	43.7	43.7	60.2
Liabilities measured at fair value:
Other accrued liabilities	0.0	0.0	0.0
Total liabilities measured at fair value	0.0	0.0	0.0
Cash equivalents	0.0	0.0	0.0
Restricted investments	0.0	0.0	0.0
Short-term investments	0.0	0.0	0.0
Long-term investments	0.0	0.0	0.0
Prepaid expenses and other current assets	0.0	0.0	0.0
Other long-term assets	43.7	43.7	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	0.0
Liabilities measured at fair value:
Derivative liabilities	0.0	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Available-for-sale securities:
Available-for-sale securities	327.4	327.4	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	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	327.4	327.4	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	0.0
Fair Value, Measurements, Recurring [Member] | Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities	49.9	49.9	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	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	49.9	49.9	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	0.0
Fair Value, Measurements, Recurring [Member] | Commercial paper [Member]
Available-for-sale securities:
Available-for-sale securities	52.4	52.4	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	0.0
Fair Value, Measurements, Recurring [Member] | Commercial paper [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities:
Available-for-sale securities	52.4	52.4	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	0.0
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities	867.3	867.3	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	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	867.3	867.3	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	0.0
Fair Value, Measurements, Recurring [Member] | Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities	38.5	38.5	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	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	38.5	38.5	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	0.0
Fair Value, Measurements, Recurring [Member] | Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities	127.4	127.4	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	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	127.4	127.4	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	0.0
Fair Value, Measurements, Recurring [Member] | Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities	413.0	413.0	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	413.0	413.0	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	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	0.0
Fair Value, Measurements, Recurring [Member] | Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities	8.1	8.1	6.2
Trading securities:
Trading securities	20.5	20.5	17.7
Fair Value, Measurements, Recurring [Member] | Mutual funds [Member] | Fair Value, Inputs, Level 1 [Member]
Available-for-sale securities:
Available-for-sale securities	8.1	8.1	6.2
Trading securities:
Trading securities	20.5	20.5	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	0.0
Trading securities:
Trading securities	0.0	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	0.0
Trading securities:
Trading securities	0.0	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities	5.3	5.3	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	5.3	5.3	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	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	0.0
Fair Value, Measurements, Recurring [Member] | US government securities [Member]
Available-for-sale securities:
Available-for-sale securities	331.9	331.9	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	320.9	320.9	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	11.0	11.0	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	0.0
Fair Value, Measurements, Recurring [Member] | Available-for-sale securities [Member]
Available-for-sale securities:
Available-for-sale securities	2,221.2	2,221.2	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	747.3	747.3	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	1,473.9	1,473.9	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	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	43.7	43.7	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
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
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	43.7	43.7	$ 60.2
Fair Value Measurements (Textuals)
Purchases of privately-held investments	$ 6.6	$ 17.1