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Unsecured Senior Notes Unsecured Senior Notes (Narrative) (Details) (USD $)
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6 Months Ended | 0 Months Ended | 6 Months Ended | 0 Months Ended | 6 Months Ended | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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Jun. 30, 2013
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Jun. 30, 2012
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Jun. 30, 2013
Senior Notes [Member]
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Apr. 11, 2013
3.279% Unsecured Senior Notes [Member]
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Jun. 30, 2013
3.279% Unsecured Senior Notes [Member]
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Jun. 27, 2013
3.916%t Unsecured Senior Notes [Member]
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Jun. 30, 2013
3.916%t Unsecured Senior Notes [Member]
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| Debt Instrument [Line Items] | |||||||||||||
| Maximum Leverage Ratio | 60.00% | ||||||||||||
| Maximum Secured Debt Leverage Ratio | 50.00% | ||||||||||||
| Minimum Interest Coverage Ratio | 1.50 | ||||||||||||
| Minimum Unencumbered Asset Value Percentage Of Unsecured Debt | 150.00% | ||||||||||||
| Proceeds from Issuance of Unsecured Debt | $ 1,194,753,000 | $ 997,790,000 | $ 492,500,000 | $ 691,900,000 | |||||||||
| Debt Instrument, Face Amount | $ 500,000,000 | $ 700,000,000 | |||||||||||
| Coupon/Stated Rate | 3.125% | 3.125% | 3.80% | 3.80% | |||||||||
| Senior Notes Pricing | 99.379% | 99.694% | |||||||||||
| Effective Percentage | 3.279% | 3.279% | [1] | 3.916% | 3.916% | [1] | |||||||
| Debt Instrument, Maturity Date | Sep. 01, 2023 | Sep. 01, 2023 | [2] | Feb. 01, 2024 | Feb. 01, 2024 | [2] | |||||||
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