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Investments (Debt Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	$ 1,960.0	$ 1,962.7
Accumulated unrealized gains (losses)	1.2	3.2
Fair value	1,961.2	1,965.9
Contractual maturity:
Due in one year or less	1,571.7
Due in one to five years	383.7
Due in five to ten years	5.8
Fair value	1,961.2	1,965.9
Government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	473.0	474.8
Accumulated unrealized gains (losses)	(0.2)
Accumulated unrealized gains (losses)		0.2
Fair value	472.8	475.0
Contractual maturity:
Fair value	472.8	475.0
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	710.4	713.2
Accumulated unrealized gains (losses)	1.2	2.3
Fair value	711.6	715.5
Contractual maturity:
Fair value	711.6	715.5
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	495.6	523.7
Accumulated unrealized gains (losses)	0.2	0.6
Fair value	495.8	524.3
Contractual maturity:
Fair value	495.8	524.3
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	260.7	251.0
Accumulated unrealized gains (losses)	0.0	0.1
Fair value	260.7	251.1
Contractual maturity:
Fair value	260.7	$ 251.1
Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	20.3
Accumulated unrealized gains (losses)	0.0
Fair value	20.3
Contractual maturity:
Fair value	$ 20.3
Weighted Average
Contractual maturity:
Weighted average life	7 months 6 days

X

- Definition

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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