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Fair Value Measurements (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Cash equivalents | $ 266,000,000 | $ 344,600,000 |
Debt securities, available for sale: | 210,600,000 | 107,100,000 |
Transfers between Level 1 and Level 2 securities | 0 | 0 |
Transfers in or out of Level 3 securities | 0 | 0 |
Goodwill and intangible asset impairment | 0 | 0 | $ 0
Fair Value, Measurements, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Total assets measured at fair value on a recurring basis | 514,600,000 | 451,700,000 |
Senior Notes |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value of outstanding notes: | 1,399,800,000 | 381,300,000 |
Senior Notes | Senior convertible notes due 2023 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value of outstanding notes: | 859,600,000 | 0 |
Senior Notes | Senior convertible notes due 2022 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Fair value of outstanding notes: | 540,200,000 | 381,300,000 |
Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Cash equivalents | 199,300,000 | 306,600,000 |
Debt securities, available for sale: | 0 | 0 |
Level 1 | Fair Value, Measurements, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Total assets measured at fair value on a recurring basis | 237,300,000 | 306,600,000 |
Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Cash equivalents | 66,700,000 | 38,000,000 |
Debt securities, available for sale: | 210,600,000 | 107,100,000 |
Level 2 | Fair Value, Measurements, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Total assets measured at fair value on a recurring basis | 277,300,000 | 145,100,000 |
Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Cash equivalents | 0 | 0 |
Debt securities, available for sale: | 0 | 0 |
Level 3 | Fair Value, Measurements, Recurring |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Total assets measured at fair value on a recurring basis | 0 | 0 |
U.S. government agencies |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt securities, available for sale: | 173,100,000 | 87,300,000 |
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 |
U.S. government agencies | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt securities, available for sale: | 173,100,000 | 87,300,000 |
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 |
Commercial paper |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt securities, available for sale: | 36,200,000 | 14,700,000 |
Commercial paper | Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt securities, available for sale: | 0 | 0 |
Commercial paper | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt securities, available for sale: | 36,200,000 | 14,700,000 |
Commercial paper | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt securities, available for sale: | 0 | 0 |
Corporate debt |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt securities, available for sale: | 1,300,000 | 5,100,000 |
Corporate debt | Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt securities, available for sale: | 0 | 0 |
Corporate debt | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Debt securities, available for sale: | 1,300,000 | 5,100,000 |
Corporate debt | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
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 [Line Items] |  |  |
Equity investment in Tandem Diabetes Care, Inc. | 38,000,000 |  |
Equity investment in Tandem Diabetes Care, Inc | Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity investment in Tandem Diabetes Care, Inc. | 38,000,000 |  |
Equity investment in Tandem Diabetes Care, Inc | Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity investment in Tandem Diabetes Care, Inc. | 0 |  |
Equity investment in Tandem Diabetes Care, Inc | Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity investment in Tandem Diabetes Care, Inc. | 0 |  |
Designated as Hedging Instrument | Foreign Exchange Forward |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Notional amount of outstanding hedge to currency risk | 60,000,000.0 |  |
Other Current Liabilities | Designated as Hedging Instrument | Foreign Exchange Forward |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Recorded net exchange derivative instrument liability | 200,000 |  |
Other Nonoperating Income (Expense) | Designated as Hedging Instrument | Foreign Exchange Forward |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Gains on forward exchange contract | $ 400,000 |  |

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