Significant Accounting Policies - FV of Other Financial Instruments (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Feb. 12, 2013	Aug. 07, 2012
Fair Value of Other Financial Instruments
Carrying value	$ 703,108	$ 707,317
Unamortized deferred financing costs	5,831
Unamortized bond premium	$ 5,685
Number of reporting segments \| item	3
Original notes - 7.625%
Fair Value of Other Financial Instruments
Carrying value	$ 421,772	420,523
Fair Value	$ 426,063	$ 420,750
Interest rate (as a percent)	7.625%	7.625%		7.625%
Unamortized deferred financing costs	$ 3,200	$ 4,500
Add-on notes - 7.625%
Fair Value of Other Financial Instruments
Carrying value	224,338	225,743
Fair Value	$ 220,550	$ 217,800
Interest rate (as a percent)	7.625%	7.625%	7.625%
Unamortized deferred financing costs	$ 1,300	$ 1,900
Unamortized bond premium	$ 5,700	$ 7,600

Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13,16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
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Amount, after accumulated amortization, of debt premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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