Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale securities:
Available-for-sale securities, estimated fair value	$ 1,792.9	$ 2,070.8
Trading securities:
Trading securities, estimated fair value	17.7	16.3
Fair Value Measurements (Textuals)
Restricted investments	29.7	41.3
Asset-backed securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	311.7	269.0
Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	9.6	10.6
Commercial Paper [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	17.7	20.3
Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	911.4	738.0
Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	16.5	24.6
Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	203.7	162.1
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	29.7	594.2
Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	6.2	4.0
Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	8.8	2.0
US Government Securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	277.6	246.0
Debt Securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	1,748.2	1,470.6
Fair Value, Measurements, Recurring [Member]
Liabilities measured at fair value:
Other accrued liabilities	(1.3)	(3.9)
Total liabilities measured at fair value	(1.3)	(3.9)
Total assets measured at fair value	1,871.2	2,134.7
Cash equivalents	3.4	576.6
Restricted investments	35.9	45.2
Short-term investments	527.1	332.2
Long-term investments	1,244.2	1,133.1
Prepaid expenses and other current assets	0.4	0.1
Other long-term assets	60.2	47.5
Fair Value, Measurements, Recurring [Member] | Foreign exchange contract [Member]
Derivative assets:
Derivative assets measured at fair value on a recurring basis	0.4	0.1
Liabilities measured at fair value:
Derivative liabilities:	(1.3)	(3.9)
Fair Value, Measurements, Recurring [Member] | Available-for-sale Securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	1,792.9	2,070.8
Fair Value, Measurements, Recurring [Member] | Asset-backed securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	311.7	269.0
Fair Value, Measurements, Recurring [Member] | Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	9.6	10.6
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	17.7	20.3
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	911.4	738.0
Fair Value, Measurements, Recurring [Member] | Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	16.5	24.6
Fair Value, Measurements, Recurring [Member] | Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	203.7	162.1
Fair Value, Measurements, Recurring [Member] | Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	29.7	594.2
Fair Value, Measurements, Recurring [Member] | Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	6.2	4.0
Trading securities:
Trading securities, estimated fair value	17.7	16.3
Fair Value, Measurements, Recurring [Member] | Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	8.8	2.0
Fair Value, Measurements, Recurring [Member] | US Government Securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	277.6	246.0
Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Trading securities:
Privately-held debt securities, estimated fair value	60.2	47.5
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Liabilities measured at fair value:
Other accrued liabilities	0.0	0.0
Total liabilities measured at fair value	0.0	0.0
Total assets measured at fair value	309.7	862.5
Cash equivalents	0.0	552.9
Restricted investments	35.9	45.2
Short-term investments	108.2	87.0
Long-term investments	165.6	177.4
Prepaid expenses and other current assets	0.0	0.0
Other long-term assets	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Foreign exchange contract [Member]
Derivative assets:
Derivative assets measured at fair value on a recurring basis	0.0	0.0
Liabilities measured at fair value:
Derivative liabilities:	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Available-for-sale Securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	292.0	846.2
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Asset-backed securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	29.7	594.2
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	6.2	4.0
Trading securities:
Trading securities, estimated fair value	17.7	16.3
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	8.8	2.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | US Government Securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	247.3	246.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Trading securities:
Privately-held debt securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Liabilities measured at fair value:
Other accrued liabilities	(1.3)	(3.9)
Total liabilities measured at fair value	(1.3)	(3.9)
Total assets measured at fair value	1,501.3	1,224.7
Cash equivalents	3.4	23.7
Restricted investments	0.0	0.0
Short-term investments	418.9	245.2
Long-term investments	1,078.6	955.7
Prepaid expenses and other current assets	0.4	0.1
Other long-term assets	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Foreign exchange contract [Member]
Derivative assets:
Derivative assets measured at fair value on a recurring basis	0.4	0.1
Liabilities measured at fair value:
Derivative liabilities:	(1.3)	(3.9)
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Available-for-sale Securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	1,500.9	1,224.6
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Asset-backed securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	311.7	269.0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	9.6	10.6
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	17.7	20.3
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	911.4	738.0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	16.5	24.6
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	203.7	162.1
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Trading securities:
Trading securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | US Government Securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	30.3	0.0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Trading securities:
Privately-held debt securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Liabilities measured at fair value:
Other accrued liabilities	0.0	0.0
Total liabilities measured at fair value	0.0	0.0
Total assets measured at fair value	60.2	47.5
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	60.2	47.5
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Foreign exchange contract [Member]
Derivative assets:
Derivative assets measured at fair value on a recurring basis	0.0	0.0
Liabilities measured at fair value:
Derivative liabilities:	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Available-for-sale Securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Asset-backed securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Government-sponsored enterprise obligations [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Trading securities:
Trading securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | US Government Securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	0.0	0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Debt Securities [Member]
Trading securities:
Privately-held debt securities, estimated fair value	60.2	$ 47.5
Fair Value Measurements (Textuals)
Purchases of privately-held investments	$ 5.3