Cash Equivalents and Investments - Available-For-Sale (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Available-for-sale debt securities:
Amortized Cost	$ 928.4	$ 1,407.3
Gross Unrealized Gains	41.7	44.3
Gross Unrealized Losses	(0.2)	(0.1)
Estimated Fair Value	969.9	1,451.5
Asset-backed securities
Available-for-sale debt securities:
Amortized Cost	70.4	76.5
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	70.5	76.7
Certificates of deposit
Available-for-sale debt securities:
Amortized Cost	28.4	32.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	28.4	32.9
Commercial paper
Available-for-sale debt securities:
Amortized Cost	81.4	89.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	81.4	89.3
Corporate debt securities
Available-for-sale debt securities:
Amortized Cost	476.0	632.0
Gross Unrealized Gains	3.4	5.5
Gross Unrealized Losses	(0.2)	(0.1)
Estimated Fair Value	479.2	637.4
Foreign government debt securities
Available-for-sale debt securities:
Amortized Cost	4.6	4.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	4.6	4.6
Time deposits
Available-for-sale debt securities:
Amortized Cost	83.1	255.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	83.1	255.6
U.S. government agency securities
Available-for-sale debt securities:
Amortized Cost	40.8	65.3
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	40.9	65.5
U.S. government securities
Available-for-sale debt securities:
Amortized Cost	124.5	232.8
Gross Unrealized Gains	0.7	1.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	125.2	233.8
Fixed Income securities
Available-for-sale debt securities:
Amortized Cost	909.2	1,389.0
Gross Unrealized Gains	4.3	6.9
Gross Unrealized Losses	(0.2)	(0.1)
Estimated Fair Value	913.3	1,395.8
Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Amortized Cost	19.2	18.3
Gross Unrealized Gains	37.4	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	56.6	55.7
Cash equivalents
Available-for-sale debt securities:
Amortized Cost	146.1	333.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	146.1	333.7
Short-term investments
Available-for-sale debt securities:
Amortized Cost	284.6	404.3
Gross Unrealized Gains	1.2	1.2
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	285.8	405.5
Long-term investments
Available-for-sale debt securities:
Amortized Cost	478.5	651.0
Gross Unrealized Gains	3.1	5.7
Gross Unrealized Losses	(0.2)	(0.1)
Estimated Fair Value	481.4	656.6
Other long-term assets
Available-for-sale debt securities:
Amortized Cost	19.2	18.3
Gross Unrealized Gains	37.4	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	$ 56.6	$ 55.7

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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