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Fair Value Measurements (Details) - USD ($) | Mar. 31, 2019 | Dec. 31, 2018 | Mar. 31, 2018
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Cash equivalents | $ 359,500,000 |  | $ 266,000,000
Debt securities, available-for-sale | 72,900,000 | $ 210,600,000 | 210,600,000
Transfers between Level 1 and Level 2 securities | 0 |  | 0
Transfers into or out of Level 3 securities | 0 |  | 0
Fair Value, Inputs, Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Cash equivalents | 309,900,000 |  | 199,300,000
Debt securities, available-for-sale | 0 |  | 0
Fair Value, Inputs, Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Cash equivalents | 49,600,000 |  | 66,700,000
Debt securities, available-for-sale | 72,900,000 |  | 210,600,000
Fair Value, Inputs, Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Cash equivalents | 0 |  | 0
Debt securities, available-for-sale | 0 |  | 0
U.S. government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 54,800,000 | 173,100,000 | 173,100,000
U.S. government agencies | Fair Value, Inputs, Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 0 |  | 0
U.S. government agencies | Fair Value, Inputs, Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 54,800,000 |  | 173,100,000
U.S. government agencies | Fair Value, Inputs, Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 0 |  | 0
Corporate debt |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 15,100,000 | 1,300,000 | 1,300,000
Corporate debt | Fair Value, Inputs, Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 0 |  | 0
Corporate debt | Fair Value, Inputs, Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 15,100,000 |  | 1,300,000
Corporate debt | Fair Value, Inputs, Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 0 |  | 0
Commercial paper |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 3,000,000 | 36,200,000 | 36,200,000
Commercial paper | Fair Value, Inputs, Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 0 |  | 0
Commercial paper | Fair Value, Inputs, Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 3,000,000 |  | 36,200,000
Commercial paper | Fair Value, Inputs, Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 0 |  | 0
Equity investment in Tandem Diabetes Care, Inc |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Equity investment in Tandem Diabetes Care, Inc. |  |  | 38,000,000
Equity investment in Tandem Diabetes Care, Inc | Fair Value, Inputs, Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Equity investment in Tandem Diabetes Care, Inc. |  |  | 38,000,000
Equity investment in Tandem Diabetes Care, Inc | Fair Value, Inputs, Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Equity investment in Tandem Diabetes Care, Inc. |  |  | 0
Equity investment in Tandem Diabetes Care, Inc | Fair Value, Inputs, Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Equity investment in Tandem Diabetes Care, Inc. |  |  | 0
Senior Notes |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Fair value of outstanding senior convertible notes: | 1,413,800,000 | 1,399,800,000 |
Senior convertible notes due 2023 | Senior Notes |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Fair value of outstanding senior convertible notes: | 876,300,000 | 859,600,000 |
Senior convertible notes due 2022 | Senior Notes |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Fair value of outstanding senior convertible notes: | 537,500,000 | $ 540,200,000 |
Foreign Exchange Forward | Designated as Hedging Instrument |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Outstanding currency hedging transactions | 0 |  |
Fair Value, Measurements, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Total assets measured at fair value on a recurring basis | 432,400,000 |  | 514,600,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Total assets measured at fair value on a recurring basis | 309,900,000 |  | 237,300,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Total assets measured at fair value on a recurring basis | 122,500,000 |  | 277,300,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Total assets measured at fair value on a recurring basis | $ 0 |  | $ 0

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