Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale debt securities:
Debt securities	$ 1,671.4	$ 2,256.8
Derivative assets:
Foreign exchange contracts	2.5	5.3
Cash equivalents
Available-for-sale debt securities:
Debt securities	290.9	936.5
Short-term investments
Available-for-sale debt securities:
Debt securities	734.2	1,067.3
Long-term investments
Available-for-sale debt securities:
Debt securities	589.8	199.0
Asset-backed securities
Available-for-sale debt securities:
Debt securities	81.4	46.5
Certificates of deposit
Available-for-sale debt securities:
Debt securities	38.6	152.9
Commercial paper
Available-for-sale debt securities:
Debt securities	168.2	393.6
Corporate debt securities
Available-for-sale debt securities:
Debt securities	605.5	413.0
Foreign government debt securities
Available-for-sale debt securities:
Debt securities	11.4	19.9
Time deposits
Available-for-sale debt securities:
Debt securities	226.3	278.6
U.S. government agency securities
Available-for-sale debt securities:
Debt securities	89.0	87.0
U.S. government securities
Available-for-sale debt securities:
Debt securities	394.5	811.3
Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Debt securities	56.5	54.0
Money market funds
Equity securities:
Equity securities	446.4	996.9
Publicly-traded equity securities
Equity securities:
Equity securities	3.8	2.8
Fair Value, Measurements, Recurring
Available-for-sale debt securities:
Debt securities	1,671.4	2,256.8
Equity securities:
Equity securities	477.0	1,024.0
Derivative assets:
Foreign exchange contracts	2.5	5.3
Total assets measured at fair value on a recurring basis	2,150.9	3,286.1
Derivative liabilities:
Foreign exchange contracts	(9.9)	(7.1)
Liability measured at non-recurring basis	(9.9)	(7.1)
Fair Value, Measurements, Recurring | Cash equivalents
Derivative assets:
Total assets measured at fair value on a recurring basis	733.2	1,921.8
Fair Value, Measurements, Recurring | Short-term investments
Derivative assets:
Total assets measured at fair value on a recurring basis	738.0	1,070.1
Fair Value, Measurements, Recurring | Long-term investments
Derivative assets:
Total assets measured at fair value on a recurring basis	589.8	199.0
Fair Value, Measurements, Recurring | Prepaid expenses and other current assets
Derivative assets:
Total assets measured at fair value on a recurring basis	6.6	16.1
Fair Value, Measurements, Recurring | Other long-term assets
Derivative assets:
Total assets measured at fair value on a recurring basis	83.3	79.1
Fair Value, Measurements, Recurring | Other accrued liabilities
Derivative liabilities:
Liability measured at non-recurring basis	(6.8)	(7.1)
Fair Value, Measurements, Recurring | Other long-term liabilities
Derivative liabilities:
Liability measured at non-recurring basis	(3.1)	0.0
Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative assets:
Foreign exchange contracts	2.5	5.3
Derivative liabilities:
Foreign exchange contracts	(6.8)	(7.1)
Fair Value, Measurements, Recurring | Interest Rate Swap
Derivative liabilities:
Foreign exchange contracts	(3.1)	0.0
Fair Value, Measurements, Recurring | Asset-backed securities
Available-for-sale debt securities:
Debt securities	81.4	46.5
Fair Value, Measurements, Recurring | Certificates of deposit
Available-for-sale debt securities:
Debt securities	38.6	152.9
Fair Value, Measurements, Recurring | Commercial paper
Available-for-sale debt securities:
Debt securities	168.2	393.6
Fair Value, Measurements, Recurring | Corporate debt securities
Available-for-sale debt securities:
Debt securities	605.5	413.0
Fair Value, Measurements, Recurring | Foreign government debt securities
Available-for-sale debt securities:
Debt securities	11.4	19.9
Fair Value, Measurements, Recurring | Time deposits
Available-for-sale debt securities:
Debt securities	226.3	278.6
Fair Value, Measurements, Recurring | U.S. government agency securities
Available-for-sale debt securities:
Debt securities	89.0	87.0
Fair Value, Measurements, Recurring | U.S. government securities
Available-for-sale debt securities:
Debt securities	394.5	811.3
Fair Value, Measurements, Recurring | Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Debt securities	56.5	54.0
Fair Value, Measurements, Recurring | Money market funds
Equity securities:
Equity securities	446.4	996.9
Fair Value, Measurements, Recurring | Mutual funds
Equity securities:
Equity securities	26.8	24.3
Fair Value, Measurements, Recurring | Publicly-traded equity securities
Equity securities:
Equity securities	3.8	2.8
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Available-for-sale debt securities:
Debt securities	318.9	352.8
Equity securities:
Equity securities	477.0	1,024.0
Derivative assets:
Foreign exchange contracts	0.0	0.0
Total assets measured at fair value on a recurring basis	795.9	1,376.8
Derivative liabilities:
Foreign exchange contracts	0.0	0.0
Liability measured at non-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	442.3	1,025.2
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	188.8	297.5
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	133.9	18.2
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	4.1	10.8
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	26.8	25.1
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Other accrued liabilities
Derivative liabilities:
Liability measured at non-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:
Liability measured at non-recurring basis	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative assets:
Foreign exchange contracts	0.0	0.0
Derivative liabilities:
Foreign exchange contracts	0.0	0.0
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Interest Rate Swap
Derivative liabilities:
Foreign exchange contracts	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	318.9	352.8
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	446.4	996.9
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Mutual funds
Equity securities:
Equity securities	26.8	24.3
Quoted Prices in Active Markets For Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Publicly-traded equity securities
Equity securities:
Equity securities	3.8	2.8
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring
Available-for-sale debt securities:
Debt securities	1,296.0	1,850.0
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Foreign exchange contracts	2.5	5.3
Total assets measured at fair value on a recurring basis	1,298.5	1,855.3
Derivative liabilities:
Foreign exchange contracts	(9.9)	(7.1)
Liability measured at non-recurring basis	(9.9)	(7.1)
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	290.9	896.6
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	549.2	772.6
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	455.9	180.8
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	2.5	5.3
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	0.0	0.0
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Other accrued liabilities
Derivative liabilities:
Liability measured at non-recurring basis	(6.8)	(7.1)
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Other long-term liabilities
Derivative liabilities:
Liability measured at non-recurring basis	(3.1)	0.0
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative assets:
Foreign exchange contracts	2.5	5.3
Derivative liabilities:
Foreign exchange contracts	(6.8)	(7.1)
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Interest Rate Swap
Derivative liabilities:
Foreign exchange contracts	(3.1)	0.0
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Asset-backed securities
Available-for-sale debt securities:
Debt securities	81.4	46.5
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Certificates of deposit
Available-for-sale debt securities:
Debt securities	38.6	152.9
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Commercial paper
Available-for-sale debt securities:
Debt securities	168.2	393.6
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Available-for-sale debt securities:
Debt securities	605.5	413.0
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Foreign government debt securities
Available-for-sale debt securities:
Debt securities	11.4	19.9
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Time deposits
Available-for-sale debt securities:
Debt securities	226.3	278.6
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | U.S. government agency securities
Available-for-sale debt securities:
Debt securities	89.0	87.0
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | U.S. government securities
Available-for-sale debt securities:
Debt securities	75.6	458.5
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
Significant Other Observable Remaining Inputs (Level 2) | Fair Value, Measurements, Recurring | Mutual funds
Equity securities:
Equity securities	0.0	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
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring
Available-for-sale debt securities:
Debt securities	56.5	54.0
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Foreign exchange contracts	0.0	0.0
Total assets measured at fair value on a recurring basis	56.5	54.0
Derivative liabilities:
Foreign exchange contracts	0.0	0.0
Liability measured at non-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	56.5	54.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Other accrued liabilities
Derivative liabilities:
Liability measured at non-recurring basis	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Other long-term liabilities
Derivative liabilities:
Liability measured at non-recurring basis	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative assets:
Foreign exchange contracts	0.0	0.0
Derivative liabilities:
Foreign exchange contracts	0.0	0.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Interest Rate Swap
Derivative liabilities:
Foreign exchange contracts	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	56.5	54.0
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Money market funds
Equity securities:
Equity securities	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
Significant Other Unobservable Remaining Inputs (Level 3) | Fair Value, Measurements, Recurring | Publicly-traded equity securities
Equity securities:
Equity securities	$ 0.0	$ 0.0