Investments (Debt Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	$ 2,942.4	$ 1,962.7
Accumulated unrealized gains (losses)	0.2	3.2
Fair value	2,942.6	1,965.9
Contractual maturity:
Due in one year or less	2,511.4
Due in one to five years	426.7
Due in five to ten years	4.5
Fair value	2,942.6	1,965.9
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	1,019.2	523.7
Accumulated unrealized gains (losses)	0.0	0.6
Fair value	1,019.2	524.3
Contractual maturity:
Fair value	1,019.2	524.3
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	846.7	713.2
Accumulated unrealized gains (losses)	0.6	2.3
Fair value	847.3	715.5
Contractual maturity:
Fair value	847.3	715.5
Government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	709.5	474.8
Accumulated unrealized gains (losses)	(0.4)	0.2
Fair value	709.1	475.0
Contractual maturity:
Fair value	709.1	475.0
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	351.1	251.0
Accumulated unrealized gains (losses)	0.0	0.1
Fair value	351.1	251.1
Contractual maturity:
Fair value	351.1	$ 251.1
Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	15.9
Accumulated unrealized gains (losses)	0.0
Fair value	15.9
Contractual maturity:
Fair value	$ 15.9
Weighted Average
Contractual maturity:
Weighted average life	7 months 6 days

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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