Cash Equivalents and Investments - Available-For-Sale (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Available-for-sale debt securities:
Amortized Cost	$ 1,582.0	$ 1,633.1
Gross Unrealized Gains	46.0	38.5
Gross Unrealized Losses	(0.2)	(0.2)
Estimated Fair Value	1,627.8	1,671.4
Asset-backed securities
Available-for-sale debt securities:
Amortized Cost	74.0	81.3
Gross Unrealized Gains	0.3	0.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	74.3	81.4
Certificates of deposit
Available-for-sale debt securities:
Amortized Cost	31.8	38.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	31.8	38.6
Commercial paper
Available-for-sale debt securities:
Amortized Cost	87.1	168.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	87.1	168.2
Corporate debt securities
Available-for-sale debt securities:
Amortized Cost	672.8	604.9
Gross Unrealized Gains	6.4	0.7
Gross Unrealized Losses	(0.2)	(0.1)
Estimated Fair Value	679.0	605.5
Foreign government debt securities
Available-for-sale debt securities:
Amortized Cost	8.0	11.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	8.0	11.4
Time deposits
Available-for-sale debt securities:
Amortized Cost	223.6	226.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	223.6	226.3
U.S. government agency securities
Available-for-sale debt securities:
Amortized Cost	58.3	89.0
Gross Unrealized Gains	0.2	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	58.5	89.0
U.S. government securities
Available-for-sale debt securities:
Amortized Cost	407.1	394.3
Gross Unrealized Gains	1.7	0.3
Gross Unrealized Losses	0.0	(0.1)
Estimated Fair Value	408.8	394.5
Fixed Income securities
Available-for-sale debt securities:
Amortized Cost	1,562.7	1,614.0
Gross Unrealized Gains	8.6	1.1
Gross Unrealized Losses	(0.2)	(0.2)
Estimated Fair Value	1,571.1	1,614.9
Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Amortized Cost	19.3	19.1
Gross Unrealized Gains	37.4	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	56.7	56.5
Cash equivalents
Available-for-sale debt securities:
Amortized Cost	356.2	290.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	356.2	290.9
Short-term investments
Available-for-sale debt securities:
Amortized Cost	492.8	733.7
Gross Unrealized Gains	1.7	0.5
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	494.5	734.2
Long-term investments
Available-for-sale debt securities:
Amortized Cost	713.7	589.4
Gross Unrealized Gains	6.9	0.6
Gross Unrealized Losses	(0.2)	(0.2)
Estimated Fair Value	720.4	589.8
Other long-term assets
Available-for-sale debt securities:
Amortized Cost	19.3	19.1
Gross Unrealized Gains	37.4	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	$ 56.7	$ 56.5

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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