Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale securities:
Available-for-sale debt securities	$ 1,811.7	$ 2,256.8
Asset-backed securities
Available-for-sale securities:
Available-for-sale debt securities	30.3	46.5
Certificates of deposit
Available-for-sale securities:
Available-for-sale debt securities	95.4	152.9
Commercial paper
Available-for-sale securities:
Available-for-sale debt securities	377.5	393.6
Corporate debt securities
Available-for-sale securities:
Available-for-sale debt securities	362.0	413.0
Foreign government debt securities
Available-for-sale securities:
Available-for-sale debt securities	38.3	19.9
Time deposits
Available-for-sale securities:
Available-for-sale debt securities	84.2	278.6
U.S. government agency securities
Available-for-sale securities:
Available-for-sale debt securities	25.6	87.0
U.S. government securities
Available-for-sale securities:
Available-for-sale debt securities	746.4	811.3
Privately-held debt and redeemable preferred stock securities
Available-for-sale securities:
Available-for-sale debt securities	52.0	54.0
Money market funds
Available-for-sale securities:
Equity securities	836.7
Equity securities		996.9
Publicly-traded equity securities
Available-for-sale securities:
Equity securities	1.8
Equity securities		2.8
Fair Value, Measurements, Recurring
Available-for-sale securities:
Equity securities	864.7
Equity securities		1,024.0
Derivative assets:
Total assets measured at fair value	2,684.8	3,286.1
Liabilities measured at fair value:
Total liabilities measured at fair value	(5.8)	(7.1)
Fair Value, Measurements, Recurring | Foreign exchange contracts
Derivative assets:
Foreign exchange contracts	8.4	5.3
Liabilities measured at fair value:
Derivative liabilities	(5.8)	(7.1)
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Equity securities	864.7
Equity securities		1,024.0
Derivative assets:
Total assets measured at fair value	1,277.6	1,376.8
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	1,355.2	1,855.3
Liabilities measured at fair value:
Total liabilities measured at fair value	(5.8)	(7.1)
Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2) | Foreign exchange contracts
Derivative assets:
Foreign exchange contracts	8.4	5.3
Liabilities measured at fair value:
Derivative liabilities	(5.8)	(7.1)
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	52.0	54.0
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	30.3	46.5
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	30.3	46.5
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	95.4	152.9
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	95.4	152.9
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	377.5	393.6
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	377.5	393.6
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	362.0	413.0
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	362.0	413.0
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	38.3	19.9
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	38.3	19.9
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	84.2	278.6
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	84.2	278.6
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	25.6	87.0
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	25.6	87.0
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	746.4	811.3
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	412.9	352.8
Fair Value, Measurements, Recurring | U.S. government securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Available-for-sale debt securities	333.5	458.5
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	52.0	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 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	52.0	54.0
Fair Value, Measurements, Recurring | Total available-for-sale debt securities
Available-for-sale securities:
Available-for-sale debt securities	1,811.7	2,256.8
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	412.9	352.8
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	1,346.8	1,850.0
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	52.0	54.0
Fair Value, Measurements, Recurring | Money market funds
Available-for-sale securities:
Equity securities	836.7
Equity securities		996.9
Fair Value, Measurements, Recurring | Money market funds | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Equity securities	836.7
Equity securities		996.9
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	26.2
Equity securities		24.3
Fair Value, Measurements, Recurring | Mutual funds | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Equity securities	26.2
Equity securities		24.3
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
Fair Value, Measurements, Recurring | Publicly-traded equity securities
Available-for-sale securities:
Equity securities	1.8
Equity securities		2.8
Fair Value, Measurements, Recurring | Publicly-traded equity securities | Quoted Prices in Active Markets For Identical Assets (Level 1)
Available-for-sale securities:
Equity securities	1.8
Equity securities		2.8
Fair Value, Measurements, Recurring | Publicly-traded equity securities | Significant Other Observable Remaining Inputs (Level 2)
Available-for-sale securities:
Equity securities	0.0
Equity securities		0.0
Fair Value, Measurements, Recurring | Publicly-traded equity securities | 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	414.4	936.5
Cash equivalents | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value	1,239.5	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	825.1	1,025.2
Cash equivalents | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	414.4	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 securities:
Available-for-sale debt securities	1,225.6	1,067.3
Short-term investments | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value	1,227.4	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	406.2	297.5
Short-term investments | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	821.2	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 securities:
Available-for-sale debt securities	119.7	199.0
Long-term investments | Fair Value, Measurements, Recurring
Derivative assets:
Total assets measured at fair value	119.7	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	8.5	18.2
Long-term investments | Fair Value, Measurements, Recurring | Significant Other Observable Remaining Inputs (Level 2)
Derivative assets:
Total assets measured at fair value	111.2	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	19.7	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	11.3	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	8.4	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	78.5	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	26.5	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	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	52.0	54.0
Other accrued liabilities | Fair Value, Measurements, Recurring
Liabilities measured at fair value:
Total liabilities measured at fair value	(5.8)	(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	(5.8)	(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