Fair Value Measurement (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Carrying amounts and estimated fair values of financial instruments
Goodwill	$ 708,285	$ 708,258	$ 747,325
Senior secured term loan
Assets and liabilities measured at fair value
Fair value percentage	99.60%
Senior secured notes
Assets and liabilities measured at fair value
Fair value percentage	101.80%
Level 3
Reconciliation of net assets and liabilities
Balance at end of period	$ 721	721	$ 721
Carrying Amount
Carrying amounts and estimated fair values of financial instruments
Acquisition contingent liability	721	721
Long-term debt	1,336,152	1,306,423
Interest rate swap	2,160	3,836
Goodwill	708,285	708,258
Fair Value
Carrying amounts and estimated fair values of financial instruments
Acquisition contingent liability	721	721
Long-term debt	1,377,941	1,316,306
Interest rate swap	2,160	3,836
Goodwill	708,285	708,258
Fair Value \| Level 2
Carrying amounts and estimated fair values of financial instruments
Long-term debt	1,377,941	1,316,306
Interest rate swap	2,160	3,836
Fair Value \| Level 3
Carrying amounts and estimated fair values of financial instruments
Acquisition contingent liability	721	721
Goodwill	$ 708,285	$ 708,258

Fair value of contingent consideration in a business combination that is classified in shareholders' equity.

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-Name Accounting Standards Codification
-Topic 820
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-Section 50
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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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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-SubTopic 10
-Section 50
-Paragraph 2
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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

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Fair value as of the balance sheet date of interest rate derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as liabilities.

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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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The estimated fair value of the debt instrument as percentage of the respective principal balance outstanding.

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