The additional liquidity that provided under a credit agreement.

No definition available.

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

- Definition

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

+ References

No definition available.

+ Details

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

Represents the amount of increase in the Line of Credit Accordion Feature borrowing capacity.

+ References

No definition available.

+ Details

Name:	cor_LineOfCreditFacilityIncreaseInAccordionFeatureBorrowingCapacity
Namespace Prefix:	cor_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ 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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_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

+ 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

+ Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
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:

- Details

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

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=cor_SeniorUnsecuredTermNotes2023Member
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:

- Details

Name:	us-gaap_DebtInstrumentAxis=cor_SeniorUnsecuredTermLoan2022Member
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:

Debt - Revolving Credit Facility (Details) $ in Thousands				3 Months Ended
Debt - Revolving Credit Facility (Details) $ in Thousands	Apr. 19, 2018 USD ($)	Feb. 02, 2016 USD ($) item	Jun. 24, 2015 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Apr. 19, 2017 USD ($)
Debt
Total principal outstanding				$ 991,500	$ 944,500
Revolving Credit Facility
Debt
Total principal outstanding				216,500	169,500
Maximum borrowing capacity				350,000
Revolving Credit Facility \| Subsequent Event
Debt
Maximum borrowing capacity	$ 450,000
Revolving Credit Facility \| LIBOR \| Minimum \| Subsequent Event
Debt
Interest rate margin (as a percent)	1.45%
Revolving Credit Facility \| LIBOR \| Maximum \| Subsequent Event
Debt
Interest rate margin (as a percent)	2.05%
Revolving Credit Facility \| Base Rate \| Minimum \| Subsequent Event
Debt
Interest rate margin (as a percent)	0.45%
Revolving Credit Facility \| Base Rate \| Maximum \| Subsequent Event
Debt
Interest rate margin (as a percent)	1.05%
Credit Agreement
Debt
Maximum borrowing capacity				600,000
Increase in accordion feature borrowing capacity				$ 150,000
Leverage ratio (as a percent)				26.80%
Secured debt ratio (as a percent)				0.00%
Fixed charge coverage ratio				8.6
Unhedged variable rate debt ratio (as a percent)				14.60%
Credit Agreement \| Subsequent Event
Debt
Additional liquidity	$ 250,000
Maximum borrowing capacity	850,000
Increase in accordion feature borrowing capacity	350,000
Maximum borrowing capacity including accordion feature	$ 1,200,000
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%
Credit Agreement \| Maximum \| Subsequent Event
Debt
Fixed charge coverage ratio	1.5
2023 Senior Unsecured Term Loan \| Subsequent Event
Debt
Total principal outstanding	$ 150,000
Term of loan facility	5 years
Unsecured term loan	$ 150,000
2023 Senior Unsecured Term Loan \| LIBOR \| Minimum \| Subsequent Event
Debt
Interest rate margin (as a percent)	1.40%
2023 Senior Unsecured Term Loan \| LIBOR \| Maximum \| Subsequent Event
Debt
Interest rate margin (as a percent)	2.00%
2023 Senior Unsecured Term Loan \| Base Rate \| Minimum \| Subsequent Event
Debt
Interest rate margin (as a percent)	0.40%
2023 Senior Unsecured Term Loan \| Base Rate \| Maximum \| Subsequent Event
Debt
Interest rate margin (as a percent)	1.00%
2020 Senior Unsecured Term Loan \| LIBOR \| Minimum \| Subsequent Event
Debt
Interest rate margin (as a percent)	1.40%
2020 Senior Unsecured Term Loan \| LIBOR \| Maximum \| Subsequent Event
Debt
Interest rate margin (as a percent)	2.00%
2020 Senior Unsecured Term Loan \| Base Rate \| Minimum \| Subsequent Event
Debt
Interest rate margin (as a percent)	0.40%
2020 Senior Unsecured Term Loan \| Base Rate \| Maximum \| Subsequent Event
Debt
Interest rate margin (as a percent)	1.00%
2020 Senior Unsecured Term Loan \| Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 150,000	150,000
2021 Senior Unsecured Term Loan \| Senior Unsecured Term Loan
Debt
Total principal outstanding				100,000	100,000
2022 Senior Unsecured Term Loan \| Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 200,000	$ 200,000
Operating partnership \| LIBOR
Debt
Interest rate margin (as a percent)				1.55%
Operating partnership \| Revolving Credit Facility \| Subsequent Event
Debt
Amount available for borrowing under the facility	$ 378,600
Operating partnership \| Credit Agreement
Debt
Number of options available for extensions \| item		1
Extension fee (as a percent)		0.10%
Maximum borrowing capacity		$ 600,000
Maximum borrowing capacity including accordion feature		800,000
Borrowing capacity at end of period				$ 350,000
Letters of credit outstanding				4,900
Amount available for borrowing under the facility				$ 128,600
Operating partnership \| Credit Agreement \| Revolving Credit Facility
Debt
Maximum borrowing capacity		350,000
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		$ 150,000	$ 150,000
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		$ 100,000
Operating partnership \| 2022 Senior Unsecured Term Loan \| LIBOR
Debt
Interest rate margin (as a percent)				1.50%
Operating partnership \| 2022 Senior Unsecured Term Loan \| LIBOR \| Minimum
Debt
Interest rate margin (as a percent)				1.50%
Operating partnership \| 2022 Senior Unsecured Term Loan \| LIBOR \| Maximum
Debt
Interest rate margin (as a percent)				2.10%
Operating partnership \| 2022 Senior Unsecured Term Loan \| Base Rate \| Minimum
Debt
Interest rate margin (as a percent)				0.50%
Operating partnership \| 2022 Senior Unsecured Term Loan \| Base Rate \| Maximum
Debt
Interest rate margin (as a percent)				1.10%
Operating partnership \| 2022 Senior Unsecured Term Loan \| Senior Unsecured Term Loan
Debt
Increase in accordion feature borrowing capacity						$ 300,000