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Debt Schedule of senior convertible notes (Details) - USD ($) | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018 | Nov. 30, 2018 | Jun. 30, 2017
Senior Notes [Abstract] |  |  |  |  |
Net carrying amount of Senior Convertible Notes due 2022 | $ 1,022,400,000 |  | $ 1,010,300,000 |  |
Senior Notes |  |  |  |  |
Senior Notes [Abstract] |  |  |  |  |
Fair value of outstanding senior convertible notes: | 1,413,800,000 |  | 1,399,800,000 |  |
Amount by which the notes' if-converted value exceeds their principal amount: | 121,700,000 | $ 125,400,000 |  |  |
Senior Notes | Zero Point Seven Five Percent Senior Convertible Notes |  |  |  |  |
Senior Notes [Abstract] |  |  |  |  |
Total net carrying amount of senior convertible notes | 1,022,400,000 |  | 1,010,300,000 |  |
Senior Notes | Senior convertible notes due 2022 |  |  |  |  |
Senior Notes [Abstract] |  |  |  |  |
Principal amount | 400,000,000.0 |  | 400,000,000.0 |  | $ 400,000,000.0
Unamortized debt discount | (47,700,000) |  | (51,100,000) |  |
Unamortized debt issuance costs | (5,900,000) |  | (6,300,000) |  |
Net carrying amount of Senior Convertible Notes due 2022 | 346,400,000 |  | 342,600,000 |  |
Fair value of outstanding senior convertible notes: | 537,500,000 |  | 540,200,000 |  |
Amount by which the notes' if-converted value exceeds their principal amount: | 121,700,000 | 125,400,000 |  |  |
Senior Notes | Senior convertible notes due 2023 |  |  |  |  |
Senior Notes [Abstract] |  |  |  |  |
Principal amount | 850,000,000.0 |  | 850,000,000.0 | $ 850,000,000 |
Unamortized debt discount | (164,000,000) |  | (171,800,000) |  |
Unamortized debt issuance costs | (10,000,000) |  | (10,500,000) |  |
Net carrying amount of Senior Convertible Notes due 2022 | 676,000,000 |  | 667,700,000 |  |
Fair value of outstanding senior convertible notes: | 876,300,000 |  | $ 859,600,000 |  |
Amount by which the notes' if-converted value exceeds their principal amount: | $ 0 | $ 0 |  |  |

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- Definition Fair value portion of borrowing which can be exchanged for a specified number of another security at the option of the issuer or the holder, for example, but not limited to, the entity's common stock. + References No definition available. + Details
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- Definition Carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Convertible Notes Payable, excluding current portion. Convertible Notes Payable is a written promise to pay a note which can be exchanged for a specified amount of another, related security, at the option of the issuer and the holder. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition The amount by which the convertible debt's if-converted value exceeds its principle amount at the balance sheet date, regardless of whether the instrument is currently convertible. This element applies to public companies only. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031898-161870 + Details
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- Definition Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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