Notes Payable (Summary Of Indebtedness) (Details) (USD $)	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Unsecured notes payable	$ 1,838,203,000us-gaap_UnsecuredDebt	$ 1,837,911,000us-gaap_UnsecuredDebt
Total secured notes payable	904,914,000us-gaap_SecuredDebt	905,628,000us-gaap_SecuredDebt
Total notes payable	2,743,100,000us-gaap_NotesPayable	2,743,500,000us-gaap_NotesPayable
5.08% Notes Due 2015 [Member] \| Senior Unsecured Notes [Member]
Unsecured notes payable	249,900,000	249,900,000
Notes payable, effective interest rate	5.08%
Debt Instrument, Maturity Date	Jun. 15, 2015
5.75% Notes Due 2017 [Member] \| Senior Unsecured Notes [Member]
Unsecured notes payable	246,500,000	246,500,000
Notes payable, effective interest rate	5.75%
Debt Instrument, Maturity Date	May 15, 2017
4.70% Notes Due 2021 [Member] \| Senior Unsecured Notes [Member]
Unsecured notes payable	249,000,000	249,000,000
Notes payable, effective interest rate	4.70%
Debt Instrument, Maturity Date	Jun. 15, 2021
3.07% Notes Due 2022 [Member] \| Senior Unsecured Notes [Member]
Unsecured notes payable	347,100,000	347,000,000
Notes payable, effective interest rate	3.07%
Debt Instrument, Maturity Date	Dec. 15, 2022
5.00% Notes Due 2023 [Member] \| Senior Unsecured Notes [Member]
Unsecured notes payable	247,900,000	247,800,000
Notes payable, effective interest rate	5.00%
Debt Instrument, Maturity Date	Jun. 15, 2023
4.27% Notes Due 2024 [Member] \| Senior Unsecured Notes [Member]
Unsecured notes payable	249,600,000	249,600,000
Notes payable, effective interest rate	4.27%
Debt Instrument, Maturity Date	Jan. 15, 2024
3.59% Notes Due 2024 [Member] \| Senior Unsecured Notes [Member]
Unsecured notes payable	248,200,000	248,100,000
Notes payable, effective interest rate	3.59%
Debt Instrument, Maturity Date	Sep. 15, 2024
Secured Notes 0.95% to 5.63% Conventional Mortgage Notes Due 2018 to 2045 [Member] \| Secured Debt [Member]
Total secured notes payable	870,600,000	870,900,000
Secured Notes 1.26% Floating Rate Tax Exempt Mortgage Note Due 2028 [Member] [Domain] \| Secured Debt [Member]
Total secured notes payable	34,300,000	34,700,000
Notes payable, effective interest rate	1.26%
Debt Instrument, Maturity Date	Jun. 15, 2028
Secured Notes 0.95% Floating Rate [Member] \| Secured Debt [Member]
Total secured notes payable	$ 175,000,000	$ 175,000,000
Minimum [Member] \| Secured Notes 0.95% to 5.63% Conventional Mortgage Notes Due 2018 to 2045 [Member] \| Secured Debt [Member]
Notes payable, effective interest rate	0.95%
Debt Instrument, Maturity Date	Oct. 01, 2018
Minimum [Member] \| Secured Notes 0.95% Floating Rate [Member] \| Secured Debt [Member]
Notes payable, effective interest rate	0.95%
Maximum [Member] \| Secured Notes 0.95% to 5.63% Conventional Mortgage Notes Due 2018 to 2045 [Member] \| Secured Debt [Member]
Notes payable, effective interest rate	5.63%
Debt Instrument, Maturity Date	Sep. 01, 2045

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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 2: 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(a)(1))

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ 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(a)(2))

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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

Reference 1: 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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-Number 210

-Section 03

-Paragraph 13, 16

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

+ References

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Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

+ References

Reference 1: 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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Paragraph 1

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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef

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-Name Regulation S-X (SX)

-Number 210

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