Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale securities:
Available-for-sale debt securities	$ 2,167.5	$ 2,418.8
Asset-backed securities
Available-for-sale securities:
Available-for-sale debt securities	71.5	286.5
Certificates of deposit
Available-for-sale securities:
Available-for-sale debt securities	96.7	83.8
Commercial paper
Available-for-sale securities:
Available-for-sale debt securities	334.7	217.1
Corporate debt securities
Available-for-sale securities:
Available-for-sale debt securities	446.7	927.0
Foreign government debt securities
Available-for-sale securities:
Available-for-sale debt securities	22.8	62.7
Time deposits
Available-for-sale securities:
Available-for-sale debt securities	309.3	239.2
U.S. government agency securities
Available-for-sale securities:
Available-for-sale debt securities	101.5	143.2
U.S. government securities
Available-for-sale securities:
Available-for-sale debt securities	730.3	406.0
Privately-held debt and redeemable preferred stock securities
Available-for-sale securities:
Available-for-sale debt securities	54.0	53.3
Money market funds
Available-for-sale securities:
Equity securities	1,126.2
Equity securities		969.8
Fair Value, Measurements, Recurring
Available-for-sale securities:
Equity securities	1,154.2
Equity securities		997.4
Derivative assets:
Total assets measured at fair value	3,325.3	3,425.4
Liabilities measured at fair value:
Total liabilities measured at fair value	(9.6)	(1.8)
Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative assets:
Foreign exchange contracts	3.6	9.2
Liabilities measured at fair value:
Derivative liabilities	(9.6)	(1.8)
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Equity securities	1,154.2
Equity securities		997.4
Derivative assets:
Total assets measured at fair value	1,467.7	1,319.8
Liabilities measured at fair value:
Total liabilities measured at fair value	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)
Available-for-sale securities:
Equity securities	0.0
Equity securities		0.0
Derivative assets:
Total assets measured at fair value	1,803.6	2,052.3
Liabilities measured at fair value:
Total liabilities measured at fair value	(9.6)	(1.8)
Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2) | Foreign exchange contracts
Derivative assets:
Foreign exchange contracts	3.6	9.2
Liabilities measured at fair value:
Derivative liabilities	(9.6)	(1.8)
Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Equity securities	0.0
Equity securities		0.0
Derivative assets:
Total assets measured at fair value	54.0	53.3
Liabilities measured at fair value:
Total liabilities measured at fair value	0.0	0.0
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 debt securities	71.5	286.5
Fair Value, Measurements, Recurring | Asset-backed securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale debt 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 debt securities	71.5	286.5
Fair Value, Measurements, Recurring | Asset-backed securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | Certificates of deposit
Available-for-sale securities:
Available-for-sale debt securities	96.7	83.8
Fair Value, Measurements, Recurring | Certificates of deposit | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale debt 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 debt securities	96.7	83.8
Fair Value, Measurements, Recurring | Certificates of deposit | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | Commercial paper
Available-for-sale securities:
Available-for-sale debt securities	334.7	217.1
Fair Value, Measurements, Recurring | Commercial paper | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale debt 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 debt securities	334.7	217.1
Fair Value, Measurements, Recurring | Commercial paper | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | Corporate debt securities
Available-for-sale securities:
Available-for-sale debt securities	446.7	927.0
Fair Value, Measurements, Recurring | Corporate debt securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale debt 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 debt securities	446.7	927.0
Fair Value, Measurements, Recurring | Corporate debt securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | Foreign government debt securities
Available-for-sale securities:
Available-for-sale debt securities	22.8	62.7
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 debt 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 debt securities	22.8	62.7
Fair Value, Measurements, Recurring | Foreign government debt securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | Time deposits
Available-for-sale securities:
Available-for-sale debt securities	309.3	239.2
Fair Value, Measurements, Recurring | Time deposits | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale debt 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 debt securities	309.3	239.2
Fair Value, Measurements, Recurring | Time deposits | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | U.S. government agency securities
Available-for-sale securities:
Available-for-sale debt securities	101.5	143.2
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 debt 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 debt securities	101.5	143.2
Fair Value, Measurements, Recurring | U.S. government agency securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | U.S. government securities
Available-for-sale securities:
Available-for-sale debt securities	730.3	406.0
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 debt securities	313.5	322.4
Fair Value, Measurements, Recurring | U.S. government securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale debt securities	416.8	83.6
Fair Value, Measurements, Recurring | U.S. government securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | Privately-held debt and redeemable preferred stock securities
Available-for-sale securities:
Available-for-sale debt securities	54.0	53.3
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:
Available-for-sale debt 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:
Available-for-sale debt 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:
Available-for-sale debt securities	54.0	53.3
Fair Value, Measurements, Recurring | Total available-for-sale debt securities
Available-for-sale securities:
Available-for-sale debt securities	2,167.5	2,418.8
Fair Value, Measurements, Recurring | Total available-for-sale debt securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Available-for-sale debt securities	313.5	322.4
Fair Value, Measurements, Recurring | Total available-for-sale debt securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale debt securities	1,800.0	2,043.1
Fair Value, Measurements, Recurring | Total available-for-sale debt securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Available-for-sale debt securities	54.0	53.3
Fair Value, Measurements, Recurring | Money market funds
Available-for-sale securities:
Equity securities	1,126.2
Equity securities		969.8
Fair Value, Measurements, Recurring | Money market funds | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Equity securities	1,126.2
Equity securities		969.8
Fair Value, Measurements, Recurring | Money market funds | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Equity securities	0.0
Equity securities		0.0
Fair Value, Measurements, Recurring | Money market funds | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Equity securities	0.0
Equity securities		0.0
Fair Value, Measurements, Recurring | Mutual funds
Available-for-sale securities:
Equity securities	28.0
Equity securities		27.6
Fair Value, Measurements, Recurring | Mutual funds | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Equity securities	28.0
Equity securities		27.6
Fair Value, Measurements, Recurring | Mutual funds | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Equity securities	0.0
Equity securities		0.0
Fair Value, Measurements, Recurring | Mutual funds | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale securities:
Equity securities	0.0
Equity securities		0.0
Cash equivalents
Available-for-sale securities:
Available-for-sale debt securities	967.2	351.0
Cash equivalents | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value	2,081.8	1,279.0
Cash equivalents | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Derivative assets:
Total assets measured at fair value	1,176.2	928.1
Cash equivalents | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	905.6	350.9
Cash equivalents | Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Derivative assets:
Total assets measured at fair value	0.0	0.0
Short-term investments
Available-for-sale securities:
Available-for-sale debt securities	887.3	1,026.1
Short-term investments | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value	887.3	1,026.1
Short-term investments | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Derivative assets:
Total assets measured at fair value	229.4	247.5
Short-term investments | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	657.9	778.6
Short-term investments | Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Derivative assets:
Total assets measured at fair value	0.0	0.0
Long-term investments
Available-for-sale securities:
Available-for-sale debt securities	259.0	988.4
Long-term investments | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value	259.0	988.4
Long-term investments | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Derivative assets:
Total assets measured at fair value	22.5	74.8
Long-term investments | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	236.5	913.6
Long-term investments | Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Derivative assets:
Total assets measured at fair value	0.0	0.0
Prepaid expenses and other current assets | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value	15.1	45.5
Prepaid expenses and other current assets | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Derivative assets:
Total assets measured at fair value	11.5	36.3
Prepaid expenses and other current assets | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	3.6	9.2
Prepaid expenses and other current assets | Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Derivative assets:
Total assets measured at fair value	0.0	0.0
Other long-term assets | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value	82.1	86.4
Other long-term assets | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Derivative assets:
Total assets measured at fair value	28.1	33.1
Other long-term assets | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	0.0	0.0
Other long-term assets | Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Derivative assets:
Total assets measured at fair value	54.0	53.3
Other accrued liabilities | Fair Value, Measurements, Recurring
Liabilities measured at fair value:
Total liabilities measured at fair value	(9.6)	(1.8)
Other accrued liabilities | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Liabilities measured at fair value:
Total liabilities measured at fair value	0.0	0.0
Other accrued liabilities | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Liabilities measured at fair value:
Total liabilities measured at fair value	(9.6)	(1.8)
Other accrued liabilities | Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Liabilities measured at fair value:
Total liabilities measured at fair value	$ 0.0	$ 0.0