Cash Equivalents and Investments - Available-For-Sale (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Available-for-sale debt securities:
Amortized Cost	$ 1,423.2	$ 1,633.1
Gross Unrealized Gains	46.7	38.5
Gross Unrealized Losses	(0.5)	(0.2)
Estimated Fair Value	1,469.4	1,671.4
Asset-backed securities
Available-for-sale debt securities:
Amortized Cost	73.9	81.3
Gross Unrealized Gains	0.5	0.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	74.4	81.4
Certificates of deposit
Available-for-sale debt securities:
Amortized Cost	34.6	38.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	34.6	38.6
Commercial paper
Available-for-sale debt securities:
Amortized Cost	74.2	168.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	74.2	168.2
Corporate debt securities
Available-for-sale debt securities:
Amortized Cost	588.8	604.9
Gross Unrealized Gains	6.5	0.7
Gross Unrealized Losses	(0.5)	(0.1)
Estimated Fair Value	594.8	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	169.9	226.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	169.9	226.3
U.S. government agency securities
Available-for-sale debt securities:
Amortized Cost	51.6	89.0
Gross Unrealized Gains	0.2	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	51.8	89.0
U.S. government securities
Available-for-sale debt securities:
Amortized Cost	402.9	394.3
Gross Unrealized Gains	2.1	0.3
Gross Unrealized Losses	0.0	(0.1)
Estimated Fair Value	405.0	394.5
Fixed Income securities
Available-for-sale debt securities:
Amortized Cost	1,403.9	1,614.0
Gross Unrealized Gains	9.3	1.1
Gross Unrealized Losses	(0.5)	(0.2)
Estimated Fair Value	1,412.7	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	307.5	290.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	307.5	290.9
Short-term investments
Available-for-sale debt securities:
Amortized Cost	391.5	733.7
Gross Unrealized Gains	1.3	0.5
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	392.8	734.2
Long-term investments
Available-for-sale debt securities:
Amortized Cost	704.9	589.4
Gross Unrealized Gains	8.0	0.6
Gross Unrealized Losses	(0.5)	(0.2)
Estimated Fair Value	712.4	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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