Cash Equivalents and Investments - Available-For-Sale (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Available-for-sale debt securities:
Amortized Cost	$ 933.3	$ 1,407.3
Gross Unrealized Gains	43.3	44.3
Gross Unrealized Losses	(0.3)	(0.1)
Estimated Fair Value	976.3	1,451.5
Cash equivalents
Available-for-sale debt securities:
Amortized Cost	93.7	333.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	93.7	333.7
Short-term investments
Available-for-sale debt securities:
Amortized Cost	327.2	404.3
Gross Unrealized Gains	1.1	1.2
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	328.3	405.5
Long-term investments
Available-for-sale debt securities:
Amortized Cost	491.2	651.0
Gross Unrealized Gains	2.3	5.7
Gross Unrealized Losses	(0.3)	(0.1)
Estimated Fair Value	493.2	656.6
Other long-term assets
Available-for-sale debt securities:
Amortized Cost	21.2	18.3
Gross Unrealized Gains	39.9	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	61.1	55.7
Asset-backed securities
Available-for-sale debt securities:
Amortized Cost	90.8	76.5
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	(0.1)	0.0
Estimated Fair Value	90.8	76.7
Certificates of deposit
Available-for-sale debt securities:
Amortized Cost	15.3	32.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	15.3	32.9
Commercial paper
Available-for-sale debt securities:
Amortized Cost	78.2	89.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	78.2	89.3
Corporate debt securities
Available-for-sale debt securities:
Amortized Cost	482.8	632.0
Gross Unrealized Gains	2.8	5.5
Gross Unrealized Losses	(0.2)	(0.1)
Estimated Fair Value	485.4	637.4
Foreign government debt securities
Available-for-sale debt securities:
Amortized Cost	7.6	4.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	7.6	4.6
Time deposits
Available-for-sale debt securities:
Amortized Cost	64.2	255.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	64.2	255.6
U.S. government agency securities
Available-for-sale debt securities:
Amortized Cost	26.6	65.3
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	26.7	65.5
U.S. government securities
Available-for-sale debt securities:
Amortized Cost	146.6	232.8
Gross Unrealized Gains	0.4	1.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	147.0	233.8
Fixed Income Securities
Available-for-sale debt securities:
Amortized Cost	912.1	1,389.0
Gross Unrealized Gains	3.4	6.9
Gross Unrealized Losses	(0.3)	(0.1)
Estimated Fair Value	915.2	1,395.8
Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Amortized Cost	21.2	18.3
Gross Unrealized Gains	39.9	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	$ 61.1	$ 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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