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Long-Term Debt - 2024 and 2020 Senior Convertible Notes, and Related Note Hedge and Warrant Transactions (Details) $ / shares in Units, shares in Millions |  |  | 3 Months Ended | 9 Months Ended |  |  |  |
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Jun. 03, 2019 USD ($) day $ / shares | May 31, 2019 USD ($) $ / shares shares | May 31, 2019 USD ($) $ / shares shares | Sep. 30, 2019 USD ($) $ / shares | Jun. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) day $ / shares shares | Sep. 30, 2018 USD ($) | May 29, 2019 USD ($) $ / shares | Dec. 31, 2018 USD ($) | Mar. 11, 2015 USD ($) | Mar. 09, 2015 shares
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Convertible note hedge |  | $ 72,000,000.0 | $ 72,000,000.0 |  |  |  |  |  |  |  |  |
Initial strike price (in usd per share) | $ / shares |  | $ 109.43 | $ 109.43 |  |  |  |  |  |  |  |  |
Aggregate proceeds warrant amount |  |  | $ 47,600,000 |  |  |  | $ 47,600,000 | $ 0 |  |  |  |
Call spread transactions |  | $ 24,400,000 | 24,400,000 |  |  |  |  |  |  |  |  |
Use of proceeds repayment of long term debt |  | 232,700,000 | $ 232,700,000 |  | $ 232,700,000 |  |  |  |  |  |  |
Payments on long-term debt |  | $ 221,100,000 |  |  | 221,100,000 |  | $ 221,091,000 | 0 |  |  |  |
Use of proceeds repurchases of common stock |  |  |  |  |  |  |  |  | $ 19,600,000 |  |  |
Closing price (in usd per share) | $ / shares |  |  |  |  |  |  |  |  | $ 62.53 |  |  |
Unwind transactions (in shares) | shares |  |  |  |  |  |  | 1.3 |  |  |  |  |
Proceeds from hedge unwind |  |  |  |  | 9,000,000.0 |  | $ 9,038,000 | 0 |  |  |  |
Payment for warrant unwind |  |  |  |  | 4,200,000 |  | 4,184,000 | 0 |  |  |  |
Proceeds for combined unwind transactions, net |  |  |  |  | 4,900,000 |  |  |  |  |  |  |
Loss on extinguishment of debt |  |  |  | $ 0 |  | $ 0 | 5,488,000 | $ 0 |  |  |  |
Debt equity component before tax |  |  |  |  | 13,000,000.0 |  |  |  |  |  |  |
Debt equity component net of tax |  |  |  |  | 10,649,000 |  |  |  |  |  |  |
Convertible Notes 2020 |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Unamortized debt discount and issuance costs |  |  |  |  | $ 3,300,000 |  |  |  |  |  |  |
Convertible Debt |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  | 494,909,000 |  |  | 494,909,000 |  |  | $ 316,000,000 |  |
Net of tax debt issuance cost |  |  |  | 6,243,000 |  |  | 6,243,000 |  |  |  |  |
Convertible Debt | Convertible Notes 2024 |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount | $ 400,000,000.0 |  |  | 400,000,000 |  |  | 400,000,000 |  |  |  |  |
Debt instrument interest rate, stated percentage | 2.00% |  |  |  |  |  |  |  |  |  |  |
Transaction fees and offering expenses | $ 391,600,000 |  |  |  |  |  |  |  |  |  |  |
Initial conversion rate | 12.3018 |  |  |  |  |  |  |  |  |  |  |
Debt conversion converted instrument amount | $ 1,000 |  |  |  |  |  | 1,000 |  |  |  |  |
Initial conversion price (in usd per share) | $ / shares | $ 81.29 |  |  |  |  |  |  |  |  |  |  |
Conversion price percentage | 130.00% |  |  |  |  |  |  |  |  |  |  |
Stock price trigger (in usd per share) | $ / shares | $ 105.68 |  |  |  |  |  |  |  |  |  |  |
Convertible threshold trading days | day | 20 |  |  |  |  |  |  |  |  |  |  |
Convertible threshold consecutive trading days | day | 30 |  |  |  |  |  |  |  |  |  |  |
Repurchase price percentage | 100.00% |  |  |  |  |  |  |  |  |  |  |
Convertible note hedge (in shares) | shares |  | 4.9 | 4.9 |  |  |  |  |  |  |  |  |
Anti-dilution adjustments of common stock (in shares) | shares |  | 4.9 | 4.9 |  |  |  |  |  |  |  |  |
Debt instrument carrying amount of liability component | $ 328,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Debt instrument carrying amount of equity component | 72,000,000.0 |  |  |  |  |  |  |  |  |  |  |
Equity component of the 2020 Notes, net of tax | $ 56,900,000 |  |  |  |  |  |  |  |  |  |  |
Effective interest rate | 6.25% |  |  |  |  |  |  |  |  |  |  |
Debt issuance directly related costs | $ 8,400,000 |  |  |  |  |  |  |  |  |  |  |
Debt issuance costs | 6,400,000 |  |  |  |  |  |  |  |  |  |  |
Debt issuance costs equity component | 1,900,000 |  |  |  |  |  |  |  |  |  |  |
Net of tax debt issuance cost | $ 1,600,000 |  |  | 6,069,000 |  |  | 6,069,000 |  |  |  |  |
Convertible Debt | Convertible Notes 2020 |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  | $ 94,909,000 |  |  | $ 94,909,000 |  |  | 316,000,000 | $ 316,000,000.0 |
Debt instrument interest rate, stated percentage | 1.50% |  |  |  |  |  |  |  |  |  | 1.50% |
Initial conversion rate |  |  |  |  |  |  | 14.1153 |  |  |  |  |
Debt conversion converted instrument amount |  |  |  |  |  |  | $ 1,000 |  |  |  |  |
Initial conversion price (in usd per share) | $ / shares |  |  |  | $ 70.85 |  |  | $ 70.85 |  |  |  |  |
Conversion price percentage |  |  |  |  |  |  | 130.00% |  |  |  |  |
Stock price trigger (in usd per share) | $ / shares |  |  |  |  |  |  | $ 92.11 |  |  |  |  |
Convertible threshold trading days | day |  |  |  |  |  |  | 20 |  |  |  |  |
Convertible threshold consecutive trading days | day |  |  |  |  |  |  | 30 |  |  |  |  |
Convertible note hedge (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  | 4.4
Anti-dilution adjustments of common stock (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  | 4.4
Initial strike price (in usd per share) | $ / shares |  |  |  | $ 86.59 |  |  | $ 86.59 |  |  |  |  |
Net of tax debt issuance cost |  |  |  | $ 174,000 |  |  | $ 174,000 |  |  | $ 1,621,000 |  |
Debt outstanding |  |  |  | $ 94,900,000 |  |  | $ 94,900,000 |  |  |  |  |

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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=119400593&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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- Definition The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585 + Details
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