Cash Equivalents and Investments - Available-For-Sale (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Available-for-sale debt securities:
Amortized Cost	$ 1,800.3	$ 2,223.6
Gross Unrealized Gains	38.5	37.4
Gross Unrealized Losses	(0.8)	(4.2)
Estimated Fair Value	1,838.0	2,256.8
Asset-backed securities
Available-for-sale debt securities:
Amortized Cost	77.4	46.8
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.0	(0.3)
Estimated Fair Value	77.5	46.5
Certificates of deposit
Available-for-sale debt securities:
Amortized Cost	70.6	152.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.1)	0.0
Estimated Fair Value	70.5	152.9
Commercial paper
Available-for-sale debt securities:
Amortized Cost	259.9	393.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	259.9	393.6
Corporate debt securities
Available-for-sale debt securities:
Amortized Cost	713.0	416.1
Gross Unrealized Gains	0.6	0.0
Gross Unrealized Losses	(0.3)	(3.1)
Estimated Fair Value	713.3	413.0
Foreign government debt securities
Available-for-sale debt securities:
Amortized Cost	15.2	20.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.1)
Estimated Fair Value	15.2	19.9
Time deposits
Available-for-sale debt securities:
Amortized Cost	17.4	278.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	17.4	278.6
U.S. government agency securities
Available-for-sale debt securities:
Amortized Cost	52.0	87.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.2)
Estimated Fair Value	52.0	87.0
U.S. government securities
Available-for-sale debt securities:
Amortized Cost	578.7	811.8
Gross Unrealized Gains	0.4	0.0
Gross Unrealized Losses	(0.4)	(0.5)
Estimated Fair Value	578.7	811.3
Fixed Income securities
Available-for-sale debt securities:
Amortized Cost	1,784.2	2,207.0
Gross Unrealized Gains	1.1	0.0
Gross Unrealized Losses	(0.8)	(4.2)
Estimated Fair Value	1,784.5	2,202.8
Privately-held debt and redeemable preferred stock securities
Available-for-sale debt securities:
Amortized Cost	16.1	16.6
Gross Unrealized Gains	37.4	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	53.5	54.0
Cash equivalents
Available-for-sale debt securities:
Amortized Cost	163.2	936.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	163.2	936.5
Short-term investments
Available-for-sale debt securities:
Amortized Cost	1,039.4	1,069.2
Gross Unrealized Gains	0.9	0.0
Gross Unrealized Losses	(0.1)	(1.9)
Estimated Fair Value	1,040.2	1,067.3
Long-term investments
Available-for-sale debt securities:
Amortized Cost	581.6	201.3
Gross Unrealized Gains	0.2	0.0
Gross Unrealized Losses	(0.7)	(2.3)
Estimated Fair Value	581.1	199.0
Other long-term assets
Available-for-sale debt securities:
Amortized Cost	16.1	16.6
Gross Unrealized Gains	37.4	37.4
Gross Unrealized Losses	0.0	0.0
Estimated Fair Value	$ 53.5	$ 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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