Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale securities:
Available-for-sale securities, estimated fair value	$ 2,782.1	$ 1,932.9
Trading securities:
Privately held investments at fair value	43.7	60.2
Total assets measured at fair value, reported as:
Restricted investments	42.9	29.7
Asset-backed securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	303.0	311.7
Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	66.1	9.6
Commercial Paper [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	147.7	17.7
Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	844.9	911.4
Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	33.9	16.5
Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	592.2	29.7
Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	8.0	6.2
Trading securities:
Trading securities, estimated fair value	21.0	17.7
Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	4.6	8.8
Time deposits [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	264.6	140.0
U.S. government agency securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	126.7	203.7
US government securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	390.4	277.6
Debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	2,177.3	1,888.2
Fair Value, Measurements, Recurring [Member]
Trading securities:
Privately held investments at fair value	43.7	60.2
Derivative assets:
Total assets measured at fair value	2,847.7	2,011.2
Derivative liabilities:
Total liabilities measured at fair value	(4.9)	(1.3)
Total assets measured at fair value, reported as:
Cash equivalents	907.0	143.4
Restricted investments	71.9	35.9
Short-term investments	752.4	527.1
Long-term investments	1,071.8	1,244.2
Prepaid expenses and other current assets	0.9	0.4
Other long-term assets	43.7	60.2
Other accrued liabilities	(4.9)	(1.3)
Fair Value, Measurements, Recurring [Member] | Foreign exchange contract [Member]
Derivative assets:
Derivative assets measured at fair value on a recurring basis	0.9	0.4
Derivative liabilities:
Derivative liabilities:	(4.9)	(1.3)
Fair Value, Measurements, Recurring [Member] | Available-for-sale Securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	2,782.1	1,932.9
Fair Value, Measurements, Recurring [Member] | Asset-backed securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	303.0	311.7
Fair Value, Measurements, Recurring [Member] | Certificates of deposit [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	66.1	9.6
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	147.7	17.7
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	844.9	911.4
Fair Value, Measurements, Recurring [Member] | Foreign government debt securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	33.9	16.5
Fair Value, Measurements, Recurring [Member] | Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	592.2	29.7
Fair Value, Measurements, Recurring [Member] | Mutual funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	8.0	6.2
Trading securities:
Trading securities, estimated fair value	21.0	17.7
Fair Value, Measurements, Recurring [Member] | Publicly-traded equity securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	4.6	8.8
Fair Value, Measurements, Recurring [Member] | Time deposits [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	264.6	140.0
Fair Value, Measurements, Recurring [Member] | U.S. government agency securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	126.7	203.7
Fair Value, Measurements, Recurring [Member] | US government securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	390.4	277.6
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Trading securities:
Privately held investments at fair value	0.0	0.0
Derivative assets:
Total assets measured at fair value	970.8	309.7
Derivative liabilities:
Total liabilities measured at fair value	0.0	0.0
Total assets measured at fair value, reported as:
Cash equivalents	549.3	0.0
Restricted investments	71.9	35.9
Short-term investments	178.1	108.2
Long-term investments	171.5	165.6
Prepaid expenses and other current assets	0.0	0.0
Other long-term assets	0.0	0.0
Other accrued liabilities	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
Derivative liabilities:
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	949.8	292.0
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] | Money market funds [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	592.2	29.7
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	8.0	6.2
Trading securities:
Trading securities, estimated fair value	21.0	17.7
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	4.6	8.8
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Time deposits [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] | U.S. government agency 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] | US government securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	345.0	247.3
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Trading securities:
Privately held investments at fair value	0.0	0.0
Derivative assets:
Total assets measured at fair value	1,833.2	1,641.3
Derivative liabilities:
Total liabilities measured at fair value	(4.9)	(1.3)
Total assets measured at fair value, reported as:
Cash equivalents	357.7	143.4
Restricted investments	0.0	0.0
Short-term investments	574.3	418.9
Long-term investments	900.3	1,078.6
Prepaid expenses and other current assets	0.9	0.4
Other long-term assets	0.0	0.0
Other accrued liabilities	(4.9)	(1.3)
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	0.9	0.4
Derivative liabilities:
Derivative liabilities:	(4.9)	(1.3)
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	1,832.3	1,640.9
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	303.0	311.7
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	66.1	9.6
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	147.7	17.7
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	844.9	911.4
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	33.9	16.5
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] | Time deposits [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	264.6	140.0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. government agency securities [Member]
Available-for-sale securities:
Available-for-sale securities, estimated fair value	126.7	203.7
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	45.4	30.3
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Trading securities:
Privately held investments at fair value	43.7	60.2
Derivative assets:
Total assets measured at fair value	43.7	60.2
Derivative liabilities:
Total liabilities measured at fair value	0.0	0.0
Total assets measured at fair value, reported as:
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	43.7	60.2
Other accrued liabilities	0.0	0.0
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
Derivative liabilities:
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] | 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] | Time deposits [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] | U.S. government agency 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]
Fair Value Measurements (Textuals)
Purchases of privately-held investments	$ 12.9