Investments (Debt Securities) (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	$ 2,983.7	$ 2,983.7
Unrealized gains	4.8	4.8
Fair value	2,988.5	2,988.5
Proceeds from sale of debt securities	1,300.0	3,800.0
Debt securities, realized gain (loss)	0.1	0.8
Contractual maturity:
Due in one year or less	2,549.3	2,549.3
Due in one to five years	432.2	432.2
Due in five to ten years	7.0	7.0
Total	2,988.5	2,988.5
Government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	998.8	998.8
Unrealized gains	0.5	0.5
Fair value	999.3	999.3
Contractual maturity:
Total	999.3	999.3
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	786.6	786.6
Unrealized gains	3.2	3.2
Fair value	789.8	789.8
Contractual maturity:
Total	789.8	789.8
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	773.1	773.1
Unrealized gains	0.9	0.9
Fair value	774.0	774.0
Contractual maturity:
Total	774.0	774.0
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	333.9	333.9
Unrealized gains	0.2	0.2
Fair value	334.1	334.1
Contractual maturity:
Total	334.1	334.1
Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	91.3	91.3
Unrealized gains	0.0	0.0
Fair value	91.3	91.3
Contractual maturity:
Total	$ 91.3	$ 91.3
Weighted Average
Debt Securities, Available-for-sale [Line Items]
Weighted average life	6 months	6 months