Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Excluding Service Costs

No definition available.

Name:	bwxt_DefinedBenefitPlanNetPeriodicBenefitCostCreditExcludingServiceCosts
Namespace Prefix:	bwxt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Defined Benefit Plan, Number of Retirees Transferred to Annuity Contract

+ References

No definition available.

+ Details

Name:	bwxt_DefinedBenefitPlanNumberofRetireesTransferredtoAnnuityContract
Namespace Prefix:	bwxt_
Data Type:	xbrli:integerItemType
Balance Type:	na
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_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanServiceCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of (increase) decrease to benefit obligation of defined benefit plan from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Includes, but is not limited to, lump-sum cash payment to participant in exchange for right to receive specified benefits, purchase of nonparticipating annuity contract and change from remeasurement.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanSettlementsBenefitObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Pension Plans and Postretirement Benefits - (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2018 USD ($) retiree	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 2,600	$ 2,300	$ 5,100	$ 4,600
Net periodic benefit (income) cost, excluding service costs		(8,800)	(6,900)	(17,700)	(13,900)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 2,405	$ 2,145	$ 4,818	$ 4,296
Subsequent Event
Defined Benefit Plan Disclosure [Line Items]
Number of retirees transferred to annuity contract \| retiree	1,300
Subsequent Event \| Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Transfer of domestic pension benefit obligation to annuity contract	$ 240,000