Represents the percentage of extension fee to be paid for exercising the option of extending the maturity term.

No definition available.

Name:	cor_DebtInstrumentExtensionOfMaturityTermExtensionFeePercentage
Namespace Prefix:	cor_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the fixed charge coverage ratio as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	cor_DebtInstrumentFixedChargeCoverageRatio
Namespace Prefix:	cor_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the leverage ratio as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	cor_DebtInstrumentLeverageRatio
Namespace Prefix:	cor_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of options to extend the mortgage loan term.

+ References

No definition available.

+ Details

Name:	cor_DebtInstrumentNumberOfOptionsForExtensionOfTermOfMaturity
Namespace Prefix:	cor_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the secured debt ratio as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	cor_DebtInstrumentSecuredDebtRatio
Namespace Prefix:	cor_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the unhedged variable rate debt ratio as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	cor_DebtInstrumentUnhedgedVariableRateDebtRatio
Namespace Prefix:	cor_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility, including the accordion feature increase, without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

No definition available.

+ Details

Name:	cor_LineOfCreditMaximumBorrowingCapacityIncludingAccordionFeature
Namespace Prefix:	cor_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Definition

Carrying value as of the balance sheet date of uncollateralized debt obligation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=cor_CreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConsolidatedEntitiesAxis=us-gaap_PartnershipInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_BaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cor_SeniorUnsecuredTermLoan2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=cor_SeniorUnsecuredTermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=cor_SeniorUnsecuredTermLoan2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt - Revolving Credit Facility (Details) $ in Millions			9 Months Ended
Debt - Revolving Credit Facility (Details) $ in Millions	Feb. 02, 2016 USD ($) item	Jun. 24, 2015 USD ($)	Sep. 30, 2017 USD ($)
Credit Agreement
Debt
Leverage ratio (as a percent)			23.80%
Secured debt ratio (as a percent)			0.00%
Fixed charge coverage ratio			7.4
Unhedged variable rate debt ratio (as a percent)			10.30%
Credit Agreement \| Minimum
Debt
Fixed charge coverage ratio			1.70
Credit Agreement \| Maximum
Debt
Leverage ratio (as a percent)			0.60%
Secured debt ratio (as a percent)			0.40%
Unhedged variable rate debt ratio (as a percent)			0.30%
Operating partnership \| LIBOR
Debt
Interest rate margin (as a percent)			1.55%
Operating partnership \| Credit Agreement
Debt
Number of options available for extensions \| item	1
Extension fee (as a percent)	0.10%
Maximum borrowing capacity	$ 600.0
Maximum borrowing capacity including accordion feature	800.0
Borrowing capacity at end of period			$ 350.0
Letters of credit outstanding			3.5
Amount available for borrowing under the facility			$ 327.5
Operating partnership \| Credit Agreement \| Revolving Credit Facility
Debt
Maximum borrowing capacity	350.0
Operating partnership \| Credit Agreement \| Revolving Credit Facility \| LIBOR \| Minimum
Debt
Interest rate margin (as a percent)			1.55%
Operating partnership \| Credit Agreement \| Revolving Credit Facility \| LIBOR \| Maximum
Debt
Interest rate margin (as a percent)			2.25%
Operating partnership \| Credit Agreement \| Revolving Credit Facility \| Base Rate \| Minimum
Debt
Interest rate margin (as a percent)			0.55%
Operating partnership \| Credit Agreement \| Revolving Credit Facility \| Base Rate \| Maximum
Debt
Interest rate margin (as a percent)			1.25%
Operating partnership \| 2020 Senior Unsecured Term Loan \| LIBOR
Debt
Interest rate margin (as a percent)			1.50%
Operating partnership \| 2020 Senior Unsecured Term Loan \| LIBOR \| Minimum
Debt
Interest rate margin (as a percent)			1.50%
Operating partnership \| 2020 Senior Unsecured Term Loan \| LIBOR \| Maximum
Debt
Interest rate margin (as a percent)			2.20%
Operating partnership \| 2020 Senior Unsecured Term Loan \| Base Rate \| Minimum
Debt
Interest rate margin (as a percent)			0.50%
Operating partnership \| 2020 Senior Unsecured Term Loan \| Base Rate \| Maximum
Debt
Interest rate margin (as a percent)			1.20%
Operating partnership \| 2020 Senior Unsecured Term Loan \| Senior Unsecured Term Loan
Debt
Term of loan facility		5 years
Unsecured term loan	$ 150.0	$ 150.0
Operating partnership \| 2021 Senior Unsecured Term Loan \| LIBOR
Debt
Interest rate margin (as a percent)			1.50%
Operating partnership \| 2021 Senior Unsecured Term Loan \| LIBOR \| Minimum
Debt
Interest rate margin (as a percent)			1.50%
Operating partnership \| 2021 Senior Unsecured Term Loan \| LIBOR \| Maximum
Debt
Interest rate margin (as a percent)			2.20%
Operating partnership \| 2021 Senior Unsecured Term Loan \| Base Rate \| Minimum
Debt
Interest rate margin (as a percent)			0.50%
Operating partnership \| 2021 Senior Unsecured Term Loan \| Base Rate \| Maximum
Debt
Interest rate margin (as a percent)			1.20%
Operating partnership \| 2021 Senior Unsecured Term Loan \| Senior Unsecured Term Loan
Debt
Term of loan facility	5 years
Unsecured term loan	$ 100.0