Commitments and Contingencies - Schedule of Restricted Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Long-term Purchase Commitment [Line Items]
Total restricted investments	$ 27,793	$ 21,886
Restricted investments, trading	26,015	13,508
Restricted investments, held-to-maturity	1,778	8,378
Workers' compensation
Long-term Purchase Commitment [Line Items]
Total restricted investments	3,131	3,100
Reclamation-related obligations
Long-term Purchase Commitment [Line Items]
Total restricted investments	$ 24,662	$ 18,786

Amount of restricted investment in debt security measured at fair value with change in fair value recognized in net income (trading).

Name:	us-gaap_DebtSecuritiesTradingRestricted
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of restricted investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-03(f))
-URI http://asc.fasb.org/extlink&oid=120401367&loc=d3e600178-122990

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesRestricted
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermPurchaseCommitmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

+ References

No definition available.

+ Details

Name:	us-gaap_RestrictedInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_CollateralAxis=ctra_WorkersCompensationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CollateralAxis=ctra_ReclamationRelatedObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: