Investments - Available-for-Sale Debt Securities (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Marketable investments classified as available-for-sale debt securities:
Amortized Cost	$ 1,833.8	$ 1,447.6
Gross Unrealized Gains	19.1	4.6
Gross Unrealized Losses	(0.1)	(0.2)
Fair Value	1,852.8	1,452.0
U.S. government and agency securities
Marketable investments classified as available-for-sale debt securities:
Amortized Cost	1,567.9	1,225.2
Gross Unrealized Gains	14.6	2.9
Gross Unrealized Losses	(0.1)	(0.2)
Fair Value	1,582.4	1,227.9
Corporate debt securities
Marketable investments classified as available-for-sale debt securities:
Amortized Cost	265.9	222.4
Gross Unrealized Gains	4.5	1.7
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 270.4	$ 224.1

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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Amount, before tax, of unrealized loss 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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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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Amount of 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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