Commitments and Contingencies - Reinsurance Agreements (Details) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($)
Insurance Loss Reserves [Roll Forward]
Claims reserves attributable to the Reinsurance Agreement at the beginning of the year	$ 0
Reinsurance premiums assumed	3,242
Claims assumed	3,934
Claims-related administrative expenses	551
(Increase) decrease in claims reserves attributable to the Reinsurance Agreement	(1,243)
Claims reserves attributable to the Reinsurance Agreement at the end of the year	$ 1,243

Insurance Loss Reserves [Roll Forward]

No definition available.

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- Definition

Policyholder Benefits And Claims Incurred, Assumed, Expenses Related To Claims

+ References

No definition available.

+ Details

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Namespace Prefix:	evh_
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- Definition

Liability as of the balance sheet date for the estimated ultimate cost of settling claims and claim adjustment expense relating to insured events that have occurred on or before the balance sheet date for those liabilities owed to another party as a result of assuming another insurer's primary obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99382578&loc=d3e14764-158437

+ Details

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Balance Type:	credit
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- Definition

Amount of written premiums assumed from other entities.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of policy benefits and costs incurred for policies assumed.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6811-158387

+ Details

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- Definition

Amount of reduction of provision for policy benefits and costs incurred for policies ceded.

+ References

+ Details

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