Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale securities:
Available-for-sale securities	$ 3,475.4	$ 2,782.1
Asset-backed securities
Available-for-sale securities:
Available-for-sale securities	287.8	303.0
Certificates of deposit
Available-for-sale securities:
Available-for-sale securities	50.0	66.1
Commercial paper
Available-for-sale securities:
Available-for-sale securities	124.0	147.7
Corporate debt securities
Available-for-sale securities:
Available-for-sale securities	834.1	844.9
Foreign government debt securities
Available-for-sale securities:
Available-for-sale securities	64.7	33.9
Money market funds
Available-for-sale securities:
Available-for-sale securities	990.7	592.2
Publicly-traded equity securities
Available-for-sale securities:
Available-for-sale securities	0.0	4.6
Time deposits
Available-for-sale securities:
Available-for-sale securities	411.4	264.6
U.S. government agency securities
Available-for-sale securities:
Available-for-sale securities	156.5	126.7
U.S. government securities
Available-for-sale securities:
Available-for-sale securities	548.0	390.4
Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value	3,552.3	2,847.7
Liabilities measured at fair value:
Other accrued liabilities	(0.6)	(4.9)
Total liabilities measured at fair value	(0.6)	(4.9)
Reported Value [Abstract]
Cash equivalents	1,588.8	907.1
Restricted investments	77.1	71.9
Short-term investments	922.0	752.3
Long-term investments	913.6	1,071.8
Prepaid expenses and other current assets	8.5	0.9
Other long-term assets	42.3	43.7
Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative assets:
Foreign exchange contracts	8.5	0.9
Liabilities measured at fair value:
Derivative liabilities	(0.6)	(4.9)
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Derivative assets:
Total assets measured at fair value	1,368.7	970.8
Liabilities measured at fair value:
Other accrued liabilities	0.0	0.0
Total liabilities measured at fair value	0.0	0.0
Reported Value [Abstract]
Cash equivalents	947.9	549.4
Restricted investments	77.1	71.9
Short-term investments	231.9	178.0
Long-term investments	111.8	171.5
Prepaid expenses and other current assets	0.0	0.0
Other long-term assets	0.0	0.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1) | Foreign exchange contracts
Derivative assets:
Foreign exchange contracts	0.0	0.0
Liabilities measured at fair value:
Derivative liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	2,141.3	1,833.2
Liabilities measured at fair value:
Other accrued liabilities	(0.6)	(4.9)
Total liabilities measured at fair value	(0.6)	(4.9)
Reported Value [Abstract]
Cash equivalents	640.9	357.7
Restricted investments	0.0	0.0
Short-term investments	690.1	574.3
Long-term investments	801.8	900.3
Prepaid expenses and other current assets	8.5	0.9
Other long-term assets	0.0	0.0
Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2) | Foreign exchange contracts
Derivative assets:
Foreign exchange contracts	8.5	0.9
Liabilities measured at fair value:
Derivative liabilities	(0.6)	(4.9)
Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Derivative assets:
Total assets measured at fair value	42.3	43.7
Liabilities measured at fair value:
Other accrued liabilities	0.0	0.0
Total liabilities measured at fair value	0.0	0.0
Reported Value [Abstract]
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	42.3	43.7
Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3) | Foreign exchange contracts
Derivative assets:
Foreign exchange contracts	0.0	0.0
Liabilities measured at fair value:
Derivative liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Asset-backed securities
Available-for-sale securities:
Available-for-sale securities	287.8	303.0
Fair Value, Measurements, Recurring | Asset-backed securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Asset-backed securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale securities	287.8	303.0
Fair Value, Measurements, Recurring | Asset-backed securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Certificates of deposit
Available-for-sale securities:
Available-for-sale securities	50.0	66.1
Fair Value, Measurements, Recurring | Certificates of deposit | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Certificates of deposit | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale securities	50.0	66.1
Fair Value, Measurements, Recurring | Certificates of deposit | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Commercial paper
Available-for-sale securities:
Available-for-sale securities	124.0	147.7
Fair Value, Measurements, Recurring | Commercial paper | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Commercial paper | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale securities	124.0	147.7
Fair Value, Measurements, Recurring | Commercial paper | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Corporate debt securities
Available-for-sale securities:
Available-for-sale securities	834.1	844.9
Fair Value, Measurements, Recurring | Corporate debt securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Corporate debt securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale securities	834.1	844.9
Fair Value, Measurements, Recurring | Corporate debt securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Foreign government debt securities
Available-for-sale securities:
Available-for-sale securities	64.7	33.9
Fair Value, Measurements, Recurring | Foreign government debt securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Foreign government debt securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale securities	64.7	33.9
Fair Value, Measurements, Recurring | Foreign government debt securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Money market funds
Available-for-sale securities:
Available-for-sale securities	990.7	592.2
Fair Value, Measurements, Recurring | Money market funds | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale securities	990.7	592.2
Fair Value, Measurements, Recurring | Money market funds | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Money market funds | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Mutual funds
Available-for-sale securities:
Available-for-sale securities	8.2	8.0
Trading securities in mutual funds	26.1	21.0
Fair Value, Measurements, Recurring | Mutual funds | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale securities	8.2	8.0
Trading securities in mutual funds	26.1	21.0
Fair Value, Measurements, Recurring | Mutual funds | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Trading securities in mutual funds	0.0	0.0
Fair Value, Measurements, Recurring | Mutual funds | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Trading securities in mutual funds	0.0	0.0
Fair Value, Measurements, Recurring | Publicly-traded equity securities
Available-for-sale securities:
Available-for-sale securities	0.0	4.6
Fair Value, Measurements, Recurring | Publicly-traded equity securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale securities	0.0	4.6
Fair Value, Measurements, Recurring | Publicly-traded equity securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Publicly-traded equity securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Time deposits
Available-for-sale securities:
Available-for-sale securities	411.4	264.6
Fair Value, Measurements, Recurring | Time deposits | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Time deposits | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale securities	411.4	264.6
Fair Value, Measurements, Recurring | Time deposits | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | U.S. government agency securities
Available-for-sale securities:
Available-for-sale securities	156.5	126.7
Fair Value, Measurements, Recurring | U.S. government agency securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | U.S. government agency securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale securities	156.5	126.7
Fair Value, Measurements, Recurring | U.S. government agency securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | U.S. government securities
Available-for-sale securities:
Available-for-sale securities	548.0	390.4
Fair Value, Measurements, Recurring | U.S. government securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale securities	343.7	345.0
Fair Value, Measurements, Recurring | U.S. government securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale securities	204.3	45.4
Fair Value, Measurements, Recurring | U.S. government securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Total available-for-sale securities
Available-for-sale securities:
Available-for-sale securities	3,475.4	2,782.1
Fair Value, Measurements, Recurring | Total available-for-sale securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale securities	1,342.6	949.8
Fair Value, Measurements, Recurring | Total available-for-sale securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale securities	2,132.8	1,832.3
Fair Value, Measurements, Recurring | Total available-for-sale securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale securities	0.0	0.0
Fair Value, Measurements, Recurring | Privately-held debt and redeemable preferred stock securities
Available-for-sale securities:
Privately-held debt and redeemable preferred stock securities	42.3	43.7
Fair Value, Measurements, Recurring | Privately-held debt and redeemable preferred stock securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Privately-held debt and redeemable preferred stock securities	0.0	0.0
Fair Value, Measurements, Recurring | Privately-held debt and redeemable preferred stock securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Privately-held debt and redeemable preferred stock securities	0.0	0.0
Fair Value, Measurements, Recurring | Privately-held debt and redeemable preferred stock securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Privately-held debt and redeemable preferred stock securities	$ 42.3	$ 43.7