Fair Value Measurements - Fair Value Hierarchy of Financial Assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 255.0	$ 266.0
Equity investment in Tandem Diabetes Care, Inc.		38.0
Debt securities, available for sale:	1,087.1	210.6
Other assets	0.7
Total assets measured at fair value on a recurring basis	1,342.8	514.6
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	110.1	199.3
Equity investment in Tandem Diabetes Care, Inc.		38.0
Debt securities, available for sale:	0.0	0.0
Other assets	0.7
Total assets measured at fair value on a recurring basis	110.8	237.3
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	144.9	66.7
Equity investment in Tandem Diabetes Care, Inc.		0.0
Debt securities, available for sale:	1,087.1	210.6
Other assets	0.0
Total assets measured at fair value on a recurring basis	1,232.0	277.3
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0.0	0.0
Equity investment in Tandem Diabetes Care, Inc.		0.0
Debt securities, available for sale:	0.0	0.0
Other assets	0.0
Total assets measured at fair value on a recurring basis	0.0	0.0
U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available for sale:	676.0	173.1
U.S. government agencies \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available for sale:	0.0	0.0
U.S. government agencies \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available for sale:	676.0	173.1
U.S. government agencies \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available for sale:	0.0	0.0
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available for sale:	248.2	36.2
Commercial paper \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available for sale:	0.0	0.0
Commercial paper \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available for sale:	248.2	36.2
Commercial paper \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available for sale:	0.0	0.0
Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available for sale:	162.9	1.3
Corporate debt \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available for sale:	0.0	0.0
Corporate debt \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available for sale:	162.9	1.3
Corporate debt \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, available for sale:	$ 0.0	$ 0.0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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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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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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