Cash Equivalents and Investments - Available for Sale Debt Securities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale debt securities:
Amortized Cost	$ 1,407.3	$ 1,633.1
Gross Unrealized Gains	44.3	38.5
Gross Unrealized Losses	(0.1)	(0.2)
Estimated Fair Value	1,451.5	1,671.4
Cash equivalents
Available-for-sale debt securities:
Amortized Cost	333.7	290.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	333.7	290.9
Short-term investments
Available-for-sale debt securities:
Amortized Cost	404.3	733.7
Gross Unrealized Gains	1.2	0.5
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	405.5	734.2
Long-term investments
Available-for-sale debt securities:
Amortized Cost	651.0	589.4
Gross Unrealized Gains	5.7	0.6
Gross Unrealized Losses	(0.1)	(0.2)
Estimated Fair Value	656.6	589.8
Other long-term assets
Available-for-sale debt securities:
Amortized Cost	18.3	19.1
Gross Unrealized Gains	37.4	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	55.7	56.5
Asset-backed securities
Available-for-sale debt securities:
Amortized Cost	76.5	81.3
Gross Unrealized Gains	0.2	0.1
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	76.7	81.4
Certificates of deposit
Available-for-sale debt securities:
Amortized Cost	32.9	38.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	32.9	38.6
Commercial paper
Available-for-sale debt securities:
Amortized Cost	89.3	168.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	89.3	168.2
Corporate debt securities
Available-for-sale debt securities:
Amortized Cost	632.0	604.9
Gross Unrealized Gains	5.5	0.7
Gross Unrealized Losses	(0.1)	(0.1)
Estimated Fair Value	637.4	605.5
Foreign government debt securities
Available-for-sale debt securities:
Amortized Cost	4.6	11.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	4.6	11.4
Time deposits
Available-for-sale debt securities:
Amortized Cost	255.6	226.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	255.6	226.3
U.S. government agency securities
Available-for-sale debt securities:
Amortized Cost	65.3	89.0
Gross Unrealized Gains	0.2	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	65.5	89.0
U.S. government securities
Available-for-sale debt securities:
Amortized Cost	232.8	394.3
Gross Unrealized Gains	1.0	0.3
Gross Unrealized Losses	0.0	(0.1)
Estimated Fair Value	233.8	394.5
Total fixed income securities
Available-for-sale debt securities:
Amortized Cost	1,389.0	1,614.0
Gross Unrealized Gains	6.9	1.1
Gross Unrealized Losses	(0.1)	(0.2)
Estimated Fair Value	1,395.8	1,614.9
Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Amortized Cost	18.3	19.1
Gross Unrealized Gains	37.4	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	$ 55.7	$ 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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