Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale debt securities:
Debt securities	$ 1,451.5	$ 1,671.4
Derivative assets:
Derivative asset, fair value, gross asset	89.0	2.5
Cash equivalents
Available-for-sale debt securities:
Debt securities	333.7	290.9
Short-term investments
Available-for-sale debt securities:
Debt securities	405.5	734.2
Long-term investments
Available-for-sale debt securities:
Debt securities	656.6	589.8
Asset-backed securities
Available-for-sale debt securities:
Debt securities	76.7	81.4
Certificates of deposit
Available-for-sale debt securities:
Debt securities	32.9	38.6
Commercial paper
Available-for-sale debt securities:
Debt securities	89.3	168.2
Corporate debt securities
Available-for-sale debt securities:
Debt securities	637.4	605.5
Foreign government debt securities
Available-for-sale debt securities:
Debt securities	4.6	11.4
Time deposits
Available-for-sale debt securities:
Debt securities	255.6	226.3
U.S. government agency securities
Available-for-sale debt securities:
Debt securities	65.5	89.0
U.S. government securities
Available-for-sale debt securities:
Debt securities	233.8	394.5
Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Debt securities	55.7	56.5
Money market funds
Equity securities:
Equity securities	536.6	446.4
Publicly-traded equity securities
Equity securities:
Equity securities	6.6	3.8
Fair Value, Measurements, Recurring
Available-for-sale debt securities:
Debt securities	1,451.5	1,671.4
Equity securities:
Equity securities	572.5	477.0
Derivative assets:
Total assets measured at fair value on a recurring basis	2,113.0	2,150.9
Derivative liabilities:
Derivative asset, fair value, gross liability	(0.5)	(9.9)
Total liabilities measured at fair value on a recurring basis	(0.5)	(9.9)
Fair Value, Measurements, Recurring | Cash equivalents
Derivative assets:
Total assets measured at fair value on a recurring basis	853.5	733.2
Fair Value, Measurements, Recurring | Short-term investments
Derivative assets:
Total assets measured at fair value on a recurring basis	412.1	738.0
Fair Value, Measurements, Recurring | Long-term investments
Derivative assets:
Total assets measured at fair value on a recurring basis	656.6	589.8
Fair Value, Measurements, Recurring | Prepaid expenses and other current assets
Derivative assets:
Total assets measured at fair value on a recurring basis	37.9	6.6
Fair Value, Measurements, Recurring | Other long-term assets
Derivative assets:
Total assets measured at fair value on a recurring basis	152.9	83.3
Fair Value, Measurements, Recurring | Other accrued liabilities
Derivative liabilities:
Total liabilities measured at fair value on a recurring basis	(0.3)	(6.8)
Fair Value, Measurements, Recurring | Other long-term liabilities
Derivative liabilities:
Total liabilities measured at fair value on a recurring basis	(0.2)	(3.1)
Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative liabilities:
Derivative asset, fair value, gross liability	(0.5)	(6.8)
Fair Value, Measurements, Recurring | Interest rate contracts
Derivative liabilities:
Derivative asset, fair value, gross liability	0.0	(3.1)
Fair Value, Measurements, Recurring | Asset-backed securities
Available-for-sale debt securities:
Debt securities	76.7	81.4
Fair Value, Measurements, Recurring | Certificates of deposit
Available-for-sale debt securities:
Debt securities	32.9	38.6
Fair Value, Measurements, Recurring | Commercial paper
Available-for-sale debt securities:
Debt securities	89.3	168.2
Fair Value, Measurements, Recurring | Corporate debt securities
Available-for-sale debt securities:
Debt securities	637.4	605.5
Fair Value, Measurements, Recurring | Foreign government debt securities
Available-for-sale debt securities:
Debt securities	4.6	11.4
Fair Value, Measurements, Recurring | Time deposits
Available-for-sale debt securities:
Debt securities	255.6	226.3
Fair Value, Measurements, Recurring | U.S. government agency securities
Available-for-sale debt securities:
Debt securities	65.5	89.0
Fair Value, Measurements, Recurring | U.S. government securities
Available-for-sale debt securities:
Debt securities	233.8	394.5
Fair Value, Measurements, Recurring | Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Debt securities	55.7	56.5
Fair Value, Measurements, Recurring | Money market funds
Equity securities:
Equity securities	536.6	446.4
Derivative assets:
Derivative asset, fair value, gross asset	38.0	2.5
Fair Value, Measurements, Recurring | Mutual funds
Equity securities:
Equity securities	29.3	26.8
Derivative assets:
Derivative asset, fair value, gross asset	51.0	0.0
Fair Value, Measurements, Recurring | Publicly-traded equity securities
Equity securities:
Equity securities	6.6	3.8
Derivative assets:
Derivative asset, fair value, gross asset	89.0	2.5
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Available-for-sale debt securities:
Debt securities	140.0	318.9
Equity securities:
Equity securities	572.5	477.0
Derivative assets:
Total assets measured at fair value on a recurring basis	712.5	795.9
Derivative liabilities:
Derivative asset, fair value, gross liability	0.0	0.0
Total liabilities measured at fair value on a recurring basis	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Cash equivalents
Derivative assets:
Total assets measured at fair value on a recurring basis	519.8	442.3
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Short-term investments
Derivative assets:
Total assets measured at fair value on a recurring basis	101.0	188.8
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Long-term investments
Derivative assets:
Total assets measured at fair value on a recurring basis	45.6	133.9
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Prepaid expenses and other current assets
Derivative assets:
Total assets measured at fair value on a recurring basis	9.9	4.1
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Other long-term assets
Derivative assets:
Total assets measured at fair value on a recurring basis	36.2	26.8
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Other accrued liabilities
Derivative liabilities:
Total liabilities measured at fair value on a recurring basis	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Other long-term liabilities
Derivative liabilities:
Total liabilities measured at fair value on a recurring basis	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative liabilities:
Derivative asset, fair value, gross liability	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Interest rate contracts
Derivative liabilities:
Derivative asset, fair value, gross liability	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Asset-backed securities
Available-for-sale debt securities:
Debt securities	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Certificates of deposit
Available-for-sale debt securities:
Debt securities	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Commercial paper
Available-for-sale debt securities:
Debt securities	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Corporate debt securities
Available-for-sale debt securities:
Debt securities	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Foreign government debt securities
Available-for-sale debt securities:
Debt securities	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Time deposits
Available-for-sale debt securities:
Debt securities	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | U.S. government agency securities
Available-for-sale debt securities:
Debt securities	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | U.S. government securities
Available-for-sale debt securities:
Debt securities	140.0	318.9
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Debt securities	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Money market funds
Equity securities:
Equity securities	536.6	446.4
Derivative assets:
Derivative asset, fair value, gross asset	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Mutual funds
Equity securities:
Equity securities	29.3	26.8
Derivative assets:
Derivative asset, fair value, gross asset	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Publicly-traded equity securities
Equity securities:
Equity securities	6.6	3.8
Derivative assets:
Derivative asset, fair value, gross asset	0.0	0.0
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring
Available-for-sale debt securities:
Debt securities	1,255.8	1,296.0
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Total assets measured at fair value on a recurring basis	1,344.8	1,298.5
Derivative liabilities:
Derivative asset, fair value, gross liability	(0.5)	(9.9)
Total liabilities measured at fair value on a recurring basis	(0.5)	(9.9)
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Cash equivalents
Derivative assets:
Total assets measured at fair value on a recurring basis	333.7	290.9
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Short-term investments
Derivative assets:
Total assets measured at fair value on a recurring basis	311.1	549.2
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Long-term investments
Derivative assets:
Total assets measured at fair value on a recurring basis	611.0	455.9
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Prepaid expenses and other current assets
Derivative assets:
Total assets measured at fair value on a recurring basis	28.0	2.5
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Other long-term assets
Derivative assets:
Total assets measured at fair value on a recurring basis	61.0	0.0
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Other accrued liabilities
Derivative liabilities:
Total liabilities measured at fair value on a recurring basis	(0.3)	(6.8)
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Other long-term liabilities
Derivative liabilities:
Total liabilities measured at fair value on a recurring basis	(0.2)	(3.1)
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative liabilities:
Derivative asset, fair value, gross liability	(0.5)	(6.8)
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Interest rate contracts
Derivative liabilities:
Derivative asset, fair value, gross liability	0.0	(3.1)
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Asset-backed securities
Available-for-sale debt securities:
Debt securities	76.7	81.4
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Certificates of deposit
Available-for-sale debt securities:
Debt securities	32.9	38.6
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Commercial paper
Available-for-sale debt securities:
Debt securities	89.3	168.2
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Available-for-sale debt securities:
Debt securities	637.4	605.5
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Foreign government debt securities
Available-for-sale debt securities:
Debt securities	4.6	11.4
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Time deposits
Available-for-sale debt securities:
Debt securities	255.6	226.3
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | U.S. government agency securities
Available-for-sale debt securities:
Debt securities	65.5	89.0
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | U.S. government securities
Available-for-sale debt securities:
Debt securities	93.8	75.6
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Debt securities	0.0	0.0
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Money market funds
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Derivative asset, fair value, gross asset	38.0	2.5
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Mutual funds
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Derivative asset, fair value, gross asset	51.0	0.0
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Publicly-traded equity securities
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Derivative asset, fair value, gross asset	89.0	2.5
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring
Available-for-sale debt securities:
Debt securities	55.7	56.5
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Total assets measured at fair value on a recurring basis	55.7	56.5
Derivative liabilities:
Derivative asset, fair value, gross liability	0.0	0.0
Total liabilities measured at fair value on a recurring basis	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Cash equivalents
Derivative assets:
Total assets measured at fair value on a recurring basis	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Short-term investments
Derivative assets:
Total assets measured at fair value on a recurring basis	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Long-term investments
Derivative assets:
Total assets measured at fair value on a recurring basis	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Prepaid expenses and other current assets
Derivative assets:
Total assets measured at fair value on a recurring basis	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Other long-term assets
Derivative assets:
Total assets measured at fair value on a recurring basis	55.7	56.5
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Other accrued liabilities
Derivative liabilities:
Total liabilities measured at fair value on a recurring basis	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Other long-term liabilities
Derivative liabilities:
Total liabilities measured at fair value on a recurring basis	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative liabilities:
Derivative asset, fair value, gross liability	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Interest rate contracts
Derivative liabilities:
Derivative asset, fair value, gross liability	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Asset-backed securities
Available-for-sale debt securities:
Debt securities	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Certificates of deposit
Available-for-sale debt securities:
Debt securities	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Commercial paper
Available-for-sale debt securities:
Debt securities	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Corporate debt securities
Available-for-sale debt securities:
Debt securities	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Foreign government debt securities
Available-for-sale debt securities:
Debt securities	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Time deposits
Available-for-sale debt securities:
Debt securities	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | U.S. government agency securities
Available-for-sale debt securities:
Debt securities	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | U.S. government securities
Available-for-sale debt securities:
Debt securities	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Debt securities	55.7	56.5
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Money market funds
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Derivative asset, fair value, gross asset	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Mutual funds
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Derivative asset, fair value, gross asset	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Publicly-traded equity securities
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Derivative asset, fair value, gross asset	$ 0.0	$ 0.0