Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Available-for-sale debt securities:
Available-for-sale debt securities	$ 1,627.8	$ 1,671.4
Derivative assets:
Derivative assets:	57.7	2.5
Asset-backed securities
Available-for-sale debt securities:
Available-for-sale debt securities	74.3	81.4
Certificates of deposit
Available-for-sale debt securities:
Available-for-sale debt securities	31.8	38.6
Commercial paper
Available-for-sale debt securities:
Available-for-sale debt securities	87.1	168.2
Corporate debt securities
Available-for-sale debt securities:
Available-for-sale debt securities	679.0	605.5
Foreign government debt securities
Available-for-sale debt securities:
Available-for-sale debt securities	8.0	11.4
Time deposits
Available-for-sale debt securities:
Available-for-sale debt securities	223.6	226.3
U.S. government agency securities
Available-for-sale debt securities:
Available-for-sale debt securities	58.5	89.0
U.S. government securities
Available-for-sale debt securities:
Available-for-sale debt securities	408.8	394.5
Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Available-for-sale debt securities	56.7	56.5
Money market funds
Equity securities:
Equity securities	580.4	446.4
Publicly-traded equity securities
Equity securities:
Equity securities	4.8	3.8
Fair Value, Measurements, Recurring
Available-for-sale debt securities:
Available-for-sale debt securities	1,627.8	1,671.4
Equity securities:
Equity securities	612.3	477.0
Derivative assets:
Derivative assets:	57.7	2.5
Total assets measured at fair value on a recurring basis	2,297.8	2,150.9
Liabilities measured at fair value:
Derivative liabilities:	(2.2)	(9.9)
Total liabilities measured at fair value on a recurring basis	(2.2)	(9.9)
Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative assets:
Derivative assets:	20.9	2.5
Liabilities measured at fair value:
Derivative liabilities:	(2.2)	(6.8)
Fair Value, Measurements, Recurring | Interest rate contracts
Derivative assets:
Derivative assets:	36.8	0.0
Liabilities measured at fair value:
Derivative liabilities:	0.0	(3.1)
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale debt securities:
Available-for-sale debt securities	214.2	318.9
Equity securities:
Equity securities	612.3	477.0
Derivative assets:
Derivative assets:	0.0	0.0
Total assets measured at fair value on a recurring basis	826.5	795.9
Liabilities measured at fair value:
Derivative liabilities:	0.0	0.0
Total liabilities measured at fair value on a recurring basis	0.0	0.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1) | Foreign exchange contracts
Derivative assets:
Derivative assets:	0.0	0.0
Liabilities measured at fair value:
Derivative liabilities:	0.0	0.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1) | Interest rate contracts
Derivative assets:
Derivative assets:	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 debt securities:
Available-for-sale debt securities	1,356.9	1,296.0
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Derivative assets:	57.7	2.5
Total assets measured at fair value on a recurring basis	1,414.6	1,298.5
Liabilities measured at fair value:
Derivative liabilities:	(2.2)	(9.9)
Total liabilities measured at fair value on a recurring basis	(2.2)	(9.9)
Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2) | Foreign exchange contracts
Derivative assets:
Derivative assets:	20.9	2.5
Liabilities measured at fair value:
Derivative liabilities:	(2.2)	(6.8)
Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2) | Interest rate contracts
Derivative assets:
Derivative assets:	36.8	0.0
Liabilities measured at fair value:
Derivative liabilities:	0.0	(3.1)
Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale debt securities:
Available-for-sale debt securities	56.7	56.5
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Derivative assets:	0.0	0.0
Total assets measured at fair value on a recurring basis	56.7	56.5
Liabilities measured at fair value:
Derivative liabilities:	0.0	0.0
Total liabilities measured at fair value on a recurring basis	0.0	0.0
Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3) | Foreign exchange contracts
Derivative assets:
Derivative assets:	0.0	0.0
Liabilities measured at fair value:
Derivative liabilities:	0.0	0.0
Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3) | Interest rate contracts
Derivative assets:
Derivative assets:	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 debt securities:
Available-for-sale debt securities	74.3	81.4
Fair Value, Measurements, Recurring | Asset-backed securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale debt 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 debt securities:
Available-for-sale debt securities	74.3	81.4
Fair Value, Measurements, Recurring | Asset-backed securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | Certificates of deposit
Available-for-sale debt securities:
Available-for-sale debt securities	31.8	38.6
Fair Value, Measurements, Recurring | Certificates of deposit | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale debt 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 debt securities:
Available-for-sale debt securities	31.8	38.6
Fair Value, Measurements, Recurring | Certificates of deposit | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | Commercial paper
Available-for-sale debt securities:
Available-for-sale debt securities	87.1	168.2
Fair Value, Measurements, Recurring | Commercial paper | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale debt 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 debt securities:
Available-for-sale debt securities	87.1	168.2
Fair Value, Measurements, Recurring | Commercial paper | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | Corporate debt securities
Available-for-sale debt securities:
Available-for-sale debt securities	679.0	605.5
Fair Value, Measurements, Recurring | Corporate debt securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale debt 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 debt securities:
Available-for-sale debt securities	679.0	605.5
Fair Value, Measurements, Recurring | Corporate debt securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | Foreign government debt securities
Available-for-sale debt securities:
Available-for-sale debt securities	8.0	11.4
Fair Value, Measurements, Recurring | Foreign government debt securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale debt 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 debt securities:
Available-for-sale debt securities	8.0	11.4
Fair Value, Measurements, Recurring | Foreign government debt securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | Time deposits
Available-for-sale debt securities:
Available-for-sale debt securities	223.6	226.3
Fair Value, Measurements, Recurring | Time deposits | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale debt 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 debt securities:
Available-for-sale debt securities	223.6	226.3
Fair Value, Measurements, Recurring | Time deposits | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | U.S. government agency securities
Available-for-sale debt securities:
Available-for-sale debt securities	58.5	89.0
Fair Value, Measurements, Recurring | U.S. government agency securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale debt 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 debt securities:
Available-for-sale debt securities	58.5	89.0
Fair Value, Measurements, Recurring | U.S. government agency securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale debt securities:
Available-for-sale debt securities	0.0	0.0
Fair Value, Measurements, Recurring | U.S. government securities
Available-for-sale debt securities:
Available-for-sale debt securities	408.8	394.5
Fair Value, Measurements, Recurring | U.S. government securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale debt securities:
Available-for-sale debt securities	214.2	318.9
Fair Value, Measurements, Recurring | U.S. government securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale debt securities:
Available-for-sale debt securities	194.6	75.6
Fair Value, Measurements, Recurring | U.S. government securities | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale debt 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 debt securities:
Available-for-sale debt securities	56.7	56.5
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 debt 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 debt 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 debt securities:
Available-for-sale debt securities	56.7	56.5
Fair Value, Measurements, Recurring | Money market funds
Equity securities:
Equity securities	580.4	446.4
Fair Value, Measurements, Recurring | Money market funds | Quoted Prices in Active Markets For Identical Assets (Level 1)
Equity securities:
Equity securities	580.4	446.4
Fair Value, Measurements, Recurring | Money market funds | Significant Other Observable Remaining Inputs (Level 2)
Equity securities:
Equity securities	0.0	0.0
Fair Value, Measurements, Recurring | Money market funds | Significant Other Unobservable Remaining Inputs (Level 3)
Equity securities:
Equity securities	0.0	0.0
Fair Value, Measurements, Recurring | Mutual funds
Equity securities:
Equity securities	27.1	26.8
Fair Value, Measurements, Recurring | Mutual funds | Quoted Prices in Active Markets For Identical Assets (Level 1)
Equity securities:
Equity securities	27.1	26.8
Fair Value, Measurements, Recurring | Mutual funds | Significant Other Observable Remaining Inputs (Level 2)
Equity securities:
Equity securities	0.0	0.0
Fair Value, Measurements, Recurring | Mutual funds | Significant Other Unobservable Remaining Inputs (Level 3)
Equity securities:
Equity securities	0.0	0.0
Fair Value, Measurements, Recurring | Publicly-traded equity securities
Equity securities:
Equity securities	4.8	3.8
Fair Value, Measurements, Recurring | Publicly-traded equity securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Equity securities:
Equity securities	4.8	3.8
Fair Value, Measurements, Recurring | Publicly-traded equity securities | Significant Other Observable Remaining Inputs (Level 2)
Equity securities:
Equity securities	0.0	0.0
Fair Value, Measurements, Recurring | Publicly-traded equity securities | Significant Other Unobservable Remaining Inputs (Level 3)
Equity securities:
Equity securities	0.0	0.0
Cash equivalents
Available-for-sale debt securities:
Available-for-sale debt securities	356.2	290.9
Cash equivalents | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value on a recurring basis	932.6	733.2
Cash equivalents | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Derivative assets:
Total assets measured at fair value on a recurring basis	576.4	442.3
Cash equivalents | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value on a recurring basis	356.2	290.9
Cash equivalents | Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Derivative assets:
Total assets measured at fair value on a recurring basis	0.0	0.0
Short-term investments
Available-for-sale debt securities:
Available-for-sale debt securities	494.5	734.2
Short-term investments | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value on a recurring basis	499.3	738.0
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 on a recurring basis	117.2	188.8
Short-term investments | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value on a recurring basis	382.1	549.2
Short-term investments | Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Derivative assets:
Total assets measured at fair value on a recurring basis	0.0	0.0
Long-term investments
Available-for-sale debt securities:
Available-for-sale debt securities	720.4	589.8
Long-term investments | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value on a recurring basis	720.4	589.8
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 on a recurring basis	101.8	133.9
Long-term investments | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value on a recurring basis	618.6	455.9
Long-term investments | Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Derivative assets:
Total assets measured at fair value on a recurring basis	0.0	0.0
Prepaid expenses and other current assets | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value on a recurring basis	19.4	6.6
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 on a recurring basis	7.2	4.1
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 on a recurring basis	12.2	2.5
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 on a recurring basis	0.0	0.0
Other long-term assets | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value on a recurring basis	126.1	83.3
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 on a recurring basis	23.9	26.8
Other long-term assets | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value on a recurring basis	45.5	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 on a recurring basis	56.7	56.5
Other accrued liabilities | Fair Value, Measurements, Recurring
Liabilities measured at fair value:
Total liabilities measured at fair value on a recurring basis	(2.0)	(6.8)
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 on a recurring basis	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 on a recurring basis	(2.0)	(6.8)
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 on a recurring basis	0.0	0.0
Other long-term liabilities | Fair Value, Measurements, Recurring
Liabilities measured at fair value:
Total liabilities measured at fair value on a recurring basis	(0.2)	(3.1)
Other long-term 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 on a recurring basis	0.0	0.0
Other long-term liabilities | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Liabilities measured at fair value:
Total liabilities measured at fair value on a recurring basis	(0.2)	(3.1)
Other long-term liabilities | Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Liabilities measured at fair value:
Total liabilities measured at fair value on a recurring basis	$ 0.0	$ 0.0