Cash Equivalents and Investments - Available for Sale Debt Securities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale debt securities:
Amortized Cost	$ 1,633.1	$ 2,223.6
Gross Unrealized Gains	38.5	37.4
Gross Unrealized Losses	(0.2)	(4.2)
Estimated Fair Value	1,671.4	2,256.8
Cash equivalents
Available-for-sale debt securities:
Amortized Cost	290.9	936.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	290.9	936.5
Short-term investments
Available-for-sale debt securities:
Amortized Cost	733.7	1,069.2
Gross Unrealized Gains	0.5	0.0
Gross Unrealized Losses	0.0	(1.9)
Estimated Fair Value	734.2	1,067.3
Long-term investments
Available-for-sale debt securities:
Amortized Cost	589.4	201.3
Gross Unrealized Gains	0.6	0.0
Gross Unrealized Losses	(0.2)	(2.3)
Estimated Fair Value	589.8	199.0
Other long-term assets
Available-for-sale debt securities:
Amortized Cost	19.1	16.6
Gross Unrealized Gains	37.4	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	56.5	54.0
Asset-backed securities
Available-for-sale debt securities:
Amortized Cost	81.3	46.8
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.0	(0.3)
Estimated Fair Value	81.4	46.5
Certificates of deposit
Available-for-sale debt securities:
Amortized Cost	38.6	152.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	38.6	152.9
Commercial paper
Available-for-sale debt securities:
Amortized Cost	168.2	393.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	168.2	393.6
Corporate debt securities
Available-for-sale debt securities:
Amortized Cost	604.9	416.1
Gross Unrealized Gains	0.7	0.0
Gross Unrealized Losses	(0.1)	(3.1)
Estimated Fair Value	605.5	413.0
Foreign government debt securities
Available-for-sale debt securities:
Amortized Cost	11.4	20.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.1)
Estimated Fair Value	11.4	19.9
Time deposits
Available-for-sale debt securities:
Amortized Cost	226.3	278.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	226.3	278.6
U.S. government agency securities
Available-for-sale debt securities:
Amortized Cost	89.0	87.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.2)
Estimated Fair Value	89.0	87.0
U.S. government securities
Available-for-sale debt securities:
Amortized Cost	394.3	811.8
Gross Unrealized Gains	0.3	0.0
Gross Unrealized Losses	(0.1)	(0.5)
Estimated Fair Value	394.5	811.3
Total fixed income securities
Available-for-sale debt securities:
Amortized Cost	1,614.0	2,207.0
Gross Unrealized Gains	1.1	0.0
Gross Unrealized Losses	(0.2)	(4.2)
Estimated Fair Value	1,614.9	2,202.8
Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Amortized Cost	19.1	16.6
Gross Unrealized Gains	37.4	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	$ 56.5	$ 54.0

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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