Notes Payable (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Total notes payable		$ 2,743.5us-gaap_NotesPayable	$ 2,530.8us-gaap_NotesPayable
Debt Instrument, Face Amount		250us-gaap_DebtInstrumentFaceAmount
Debt Instrument, Interest Rate, Stated Percentage		3.50%
Discounted notes payable face amount	99.231%cpt_DiscountedNotesPayableFaceAmount
Notes payable, effective interest rate		3.59%
Proceeds from Debt, Net of Issuance Costs	245.7
Weighted average interest rate on floating rate debt		4.40%us-gaap_DebtWeightedAverageInterestRate
Weighted average maturity of indebtedness (including unsecured line of credit) (in years)		6.3
Letter Of Credit [Member]
Maximum borrowing capacity under unsecured credit facility		100
Outstanding letters of credit under credit facility		6.4
Floating Rate Notes Payable [Member]
Total notes payable		209.7	211.3
Weighted average interest rate on floating rate debt		1.00%	1.00%
Line of Credit [Member]
Amount of current borrowing capacity under unsecured credit facility		500
Unsecured Credit Facility, Expiration Date		Sep. 22, 2015
Optional extension period of unsecured credit facility		September 2016
Maximum borrowing capacity under unsecured credit facility		750
Maximum term of bid rate loans (days)		180
Lesser of amount stated or the amount available under the line of credit		250
Available amount under unsecured credit facility		$ 493.6

The discount percentage applied to the new notes payable face amount for the public offering of the debt.

No definition available.

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- Definition

Option to extend the period of unsecured credit facility.

+ References

No definition available.

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- Definition

Maximum term of bid rate loans in days related to the line of credit. Advances under the line of credit may be priced at the scheduled rates, or we may enter into bid rate loans with participating banks at rates below the scheduled rates

+ References

No definition available.

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Advances under the line of credit may be priced at the scheduled rates, or we may enter into bid rate loans with participating banks at rates below the scheduled rates. These bid rate loans have terms of 180 days or less and may not exceed the lesser of $250 million or the remaining amount available under the line of credit.

+ References

No definition available.

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- Definition

Weighted Average Maturity Of Indebtedness Including Unsecured Line Of Credit

+ References

No definition available.

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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-Number 210

-Section 02

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- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

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-Subparagraph (SX 210.7-03.16)

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- 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.

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-SubTopic 10

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-Number 210

-Section 02

-Paragraph 19, 22

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- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

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- 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

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- Definition

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

+ References

+ Details

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- Definition

Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

+ References

+ Details

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- Definition

The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

+ References

No definition available.

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