Cash Equivalents and Investments - Available-For-Sale (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Available-for-sale debt securities:
Amortized Cost	$ 1,226.8	$ 1,633.1
Gross Unrealized Gains	41.1	38.5
Gross Unrealized Losses	(6.0)	(0.2)
Estimated Fair Value	1,261.9	1,671.4
Asset-backed securities
Available-for-sale debt securities:
Amortized Cost	82.8	81.3
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	(0.6)	0.0
Estimated Fair Value	82.2	81.4
Certificates of deposit
Available-for-sale debt securities:
Amortized Cost	36.8	38.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	36.8	38.6
Commercial paper
Available-for-sale debt securities:
Amortized Cost	109.3	168.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	109.3	168.2
Corporate debt securities
Available-for-sale debt securities:
Amortized Cost	551.6	604.9
Gross Unrealized Gains	0.6	0.7
Gross Unrealized Losses	(5.4)	(0.1)
Estimated Fair Value	546.8	605.5
Foreign government debt securities
Available-for-sale debt securities:
Amortized Cost	11.2	11.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	11.2	11.4
Time deposits
Available-for-sale debt securities:
Amortized Cost	0.0	226.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	0.0	226.3
U.S. government agency securities
Available-for-sale debt securities:
Amortized Cost	64.5	89.0
Gross Unrealized Gains	0.3	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	64.8	89.0
U.S. government securities
Available-for-sale debt securities:
Amortized Cost	351.5	394.3
Gross Unrealized Gains	2.8	0.3
Gross Unrealized Losses	0.0	(0.1)
Estimated Fair Value	354.3	394.5
Fixed Income securities
Available-for-sale debt securities:
Amortized Cost	1,207.7	1,614.0
Gross Unrealized Gains	3.7	1.1
Gross Unrealized Losses	(6.0)	(0.2)
Estimated Fair Value	1,205.4	1,614.9
Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Amortized Cost	19.1	19.1
Gross Unrealized Gains	37.4	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	56.5	56.5
Cash equivalents
Available-for-sale debt securities:
Amortized Cost	76.2	290.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	76.2	290.9
Short-term investments
Available-for-sale debt securities:
Amortized Cost	520.8	733.7
Gross Unrealized Gains	1.1	0.5
Gross Unrealized Losses	(1.0)	0.0
Estimated Fair Value	520.9	734.2
Long-term investments
Available-for-sale debt securities:
Amortized Cost	610.7	589.4
Gross Unrealized Gains	2.6	0.6
Gross Unrealized Losses	(5.0)	(0.2)
Estimated Fair Value	608.3	589.8
Other long-term assets
Available-for-sale debt securities:
Amortized Cost	19.1	19.1
Gross Unrealized Gains	37.4	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	$ 56.5	$ 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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