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Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2016
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Total long-term debt | $ 2,757,481 | $ 2,097,606 |
Line of Credit [Member] | Revolving Credit Facility [Member] | Senior Secured Revolving Line of Credit [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Total principal of senior unsecured notes and revolving line of credit | 651,000 | 70,000 |
Line of Credit [Member] | Revolving Credit Facility [Member] | OMP Revolving Line of Credit [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Total principal of senior unsecured notes and revolving line of credit | 165,000 | 78,000 |
Senior Notes [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Total principal of senior unsecured notes and revolving line of credit | 2,039,409 | 2,052,950 |
Less: unamortized deferred financing costs on senior unsecured notes | $ (22,889) | (22,956) |
Senior Notes [Member] | 7.25% senior unsecured notes due February 1, 2019 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate (as percent) | 7.25% |  |
Total principal of senior unsecured notes and revolving line of credit | $ 0 | 54,275 |
Senior Notes [Member] | 6.50% senior unsecured notes due November 1, 2021 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate (as percent) | 6.50% |  |
Total principal of senior unsecured notes and revolving line of credit | $ 71,835 | 395,501 |
Senior Notes [Member] | 6.875% senior unsecured notes due March 15, 2022 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate (as percent) | 6.875% |  |
Total principal of senior unsecured notes and revolving line of credit | $ 901,480 | 937,080 |
Senior Notes [Member] | 6.875% senior unsecured notes due January 15, 2023 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate (as percent) | 6.875% |  |
Total principal of senior unsecured notes and revolving line of credit | $ 366,094 | 366,094 |
Senior Notes [Member] | 6.25% senior unsecured notes due May 1, 2026 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate (as percent) | 6.25% |  |
Total principal of senior unsecured notes and revolving line of credit | $ 400,000 | 0 |
Senior Notes [Member] | 2.625% senior convertible notes due September 15, 2023 [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Stated interest rate (as percent) | 2.625% |  |
Total principal of senior unsecured notes and revolving line of credit | $ 300,000 | 300,000 |
Less: unamortized debt discount on senior unsecured convertible notes | $ (75,039) | $ (80,388) | $ (93,200)

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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + 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)(1) -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(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 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: 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 + Details
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