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Fair Value Measurements (Details) - USD ($) | Sep. 30, 2019 | Dec. 31, 2018 | Sep. 30, 2018
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Cash equivalents | $ 301,900,000 | $ 266,000,000.0 |
Debt securities, available-for-sale | 1,034,300,000 | 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 | 256,900,000 | 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 | 45,000,000.0 | 66,700,000 |
Debt securities, available-for-sale | 1,034,300,000 | 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 | 699,000,000.0 | 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 | 699,000,000.0 | 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 |
Commercial paper |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 277,900,000 | 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 | 277,900,000 | 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 |
Corporate debt |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |  |
Debt securities, available-for-sale | 57,400,000 | 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 | 57,400,000 | 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 |
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 | 1,336,200,000 | 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 | 256,900,000 | 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 | 1,079,300,000 | 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 |
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. |  | 38,000,000.0 |
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. |  | 38,000,000.0 |
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 |

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