Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Available-for-sale debt securities:
Available-for-sale debt securities	$ 1,838.0	$ 2,256.8
Asset-backed securities
Available-for-sale debt securities:
Available-for-sale debt securities	77.5	46.5
Certificates of deposit
Available-for-sale debt securities:
Available-for-sale debt securities	70.5	152.9
Commercial paper
Available-for-sale debt securities:
Available-for-sale debt securities	259.9	393.6
Corporate debt securities
Available-for-sale debt securities:
Available-for-sale debt securities	713.3	413.0
Foreign government debt securities
Available-for-sale debt securities:
Available-for-sale debt securities	15.2	19.9
Time deposits
Available-for-sale debt securities:
Available-for-sale debt securities	17.4	278.6
U.S. government agency securities
Available-for-sale debt securities:
Available-for-sale debt securities	52.0	87.0
U.S. government securities
Available-for-sale debt securities:
Available-for-sale debt securities	578.7	811.3
Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Available-for-sale debt securities	53.5	54.0
Money market funds
Equity securities:
Equity securities	611.9	996.9
Publicly-traded equity securities
Equity securities:
Equity securities	0.6	2.8
Fair Value, Measurements, Recurring
Available-for-sale debt securities:
Available-for-sale debt securities	1,838.0	2,256.8
Equity securities:
Equity securities	638.9	1,024.0
Derivative assets:
Derivative assets	3.8	5.3
Total assets measured at fair value	2,480.7	3,286.1
Liabilities measured at fair value:
Derivative liabilities	(14.4)	(7.1)
Total liabilities measured at fair value	(14.4)	(7.1)
Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative assets:
Derivative assets	2.5	5.3
Liabilities measured at fair value:
Derivative liabilities	(14.2)	(7.1)
Fair Value, Measurements, Recurring | Interest rate swap contracts
Derivative assets:
Derivative assets	1.3	0.0
Liabilities measured at fair value:
Derivative liabilities	(0.2)	0.0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale debt securities:
Available-for-sale debt securities	454.6	352.8
Equity securities:
Equity securities	638.9	1,024.0
Derivative assets:
Derivative assets	0.0	0.0
Total assets measured at fair value	1,093.5	1,376.8
Liabilities measured at fair value:
Derivative liabilities	0.0	0.0
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:
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 swap 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	1,329.9	1,850.0
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Derivative assets	3.8	5.3
Total assets measured at fair value	1,333.7	1,855.3
Liabilities measured at fair value:
Derivative liabilities	(14.4)	(7.1)
Total liabilities measured at fair value	(14.4)	(7.1)
Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2) | Foreign exchange contracts
Derivative assets:
Derivative assets	2.5	5.3
Liabilities measured at fair value:
Derivative liabilities	(14.2)	(7.1)
Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2) | Interest rate swap contracts
Derivative assets:
Derivative assets	1.3	0.0
Liabilities measured at fair value:
Derivative liabilities	(0.2)	0.0
Fair Value, Measurements, Recurring | Significant Other Unobservable Remaining Inputs (Level 3)
Available-for-sale debt securities:
Available-for-sale debt securities	53.5	54.0
Equity securities:
Equity securities	0.0	0.0
Derivative assets:
Derivative assets	0.0	0.0
Total assets measured at fair value	53.5	54.0
Liabilities measured at fair value:
Derivative liabilities	0.0	0.0
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:
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 swap 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	77.5	46.5
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	77.5	46.5
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	70.5	152.9
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	70.5	152.9
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	259.9	393.6
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	259.9	393.6
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	713.3	413.0
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	713.3	413.0
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	15.2	19.9
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	15.2	19.9
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	17.4	278.6
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	17.4	278.6
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	52.0	87.0
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	52.0	87.0
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	578.7	811.3
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	454.6	352.8
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	124.1	458.5
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	53.5	54.0
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	53.5	54.0
Fair Value, Measurements, Recurring | Money market funds
Equity securities:
Equity securities	611.9	996.9
Fair Value, Measurements, Recurring | Money market funds | Quoted Prices in Active Markets For Identical Assets (Level 1)
Equity securities:
Equity securities	611.9	996.9
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	26.4	24.3
Fair Value, Measurements, Recurring | Mutual funds | Quoted Prices in Active Markets For Identical Assets (Level 1)
Equity securities:
Equity securities	26.4	24.3
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	0.6	2.8
Fair Value, Measurements, Recurring | Publicly-traded equity securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Equity securities:
Equity securities	0.6	2.8
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	163.2	936.5
Cash equivalents | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value	770.8	1,921.8
Cash equivalents | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Derivative assets:
Total assets measured at fair value	621.5	1,025.2
Cash equivalents | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	149.3	896.6
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 debt securities:
Available-for-sale debt securities	1,040.2	1,067.3
Short-term investments | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value	1,040.8	1,070.1
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	221.7	297.5
Short-term investments | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	819.1	772.6
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 debt securities:
Available-for-sale debt securities	581.1	199.0
Long-term investments | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value	581.1	199.0
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	219.6	18.2
Long-term investments | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	361.5	180.8
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	7.7	16.1
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	5.2	10.8
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	2.5	5.3
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	80.3	79.1
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	25.5	25.1
Other long-term assets | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	1.3	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	53.5	54.0
Other accrued liabilities | Fair Value, Measurements, Recurring
Liabilities measured at fair value:
Total liabilities measured at fair value	(14.2)	(7.1)
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	(14.2)	(7.1)
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
Other long-term liabilities | Fair Value, Measurements, Recurring
Liabilities measured at fair value:
Total liabilities measured at fair value	(0.2)	0.0
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	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	(0.2)	0.0
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	$ 0.0	$ 0.0