Employee Benefit Plans - Projected future postretirement benefit payments and future receipts from the federal subsidy (Detail) - Other Postretirement Benefits Plan [Member] $ in Millions	Dec. 31, 2017 USD ($)
Defined benefit plan, expected future benefit payment
2018	$ 2.5
2019	2.2
2020	2.1
2021	2.0
2022	1.9
Next five years	7.3
Defined benefit plan, expected future receipts from federal subsidy
2018	0.1
2019	0.1
2020	0.1
2021	0.1
2022	0.1
Next five years	$ 0.3

Defined Benefit Plan, Expected Future Receipts From Federal Subsidy [Abstract]

No definition available.

Name:	cday_DefinedBenefitPlanExpectedFutureReceiptsFromFederalSubsidyAbstract
Namespace Prefix:	cday_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined benefit plan expected future receipts from federal subsidy after year five.

+ References

No definition available.

+ Details

Name:	cday_DefinedBenefitPlanExpectedFutureReceiptsFromFederalSubsidyAfterYearFive
Namespace Prefix:	cday_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Defined benefit plan expected future receipts from federal subsidy next twelve months.

+ References

No definition available.

+ Details

Name:	cday_DefinedBenefitPlanExpectedFutureReceiptsFromFederalSubsidyNextTwelveMonths
Namespace Prefix:	cday_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Defined benefit plan expected future receipts from federal subsidy year five.

+ References

No definition available.

+ Details

Name:	cday_DefinedBenefitPlanExpectedFutureReceiptsFromFederalSubsidyYearFive
Namespace Prefix:	cday_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Defined benefit plan expected future receipts from federal subsidy year four.

+ References

No definition available.

+ Details

Name:	cday_DefinedBenefitPlanExpectedFutureReceiptsFromFederalSubsidyYearFour
Namespace Prefix:	cday_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Defined benefit plan expected future receipts from federal subsidy year three.

+ References

No definition available.

+ Details

Name:	cday_DefinedBenefitPlanExpectedFutureReceiptsFromFederalSubsidyYearThree
Namespace Prefix:	cday_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Defined benefit plan expected future receipts from federal subsidy year two.

+ References

No definition available.

+ Details

Name:	cday_DefinedBenefitPlanExpectedFutureReceiptsFromFederalSubsidyYearTwo
Namespace Prefix:	cday_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Amount of benefits for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following latest fiscal year.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of benefits for defined benefit plan expected to be paid in next fiscal year following latest fiscal year.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of benefits for defined benefit plan expected to be paid in fifth fiscal year following latest fiscal year.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of benefits for defined benefit plan expected to be paid in fourth fiscal year following latest fiscal year.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of benefits for defined benefit plan expected to be paid in third fiscal year following latest fiscal year.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of benefits for defined benefit plan expected to be paid in second fiscal year following latest fiscal year.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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