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Borrowing Arrangements (Details 1) (USD $)
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12 Months Ended | 1 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 12 Months Ended | |||||||||||
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Dec. 31, 2012
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Dec. 31, 2011
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Dec. 31, 2010
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Dec. 31, 2012
Eurocurrency rate loans
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Dec. 31, 2012
Base rate loans
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Dec. 31, 2011
Line of Credit
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Dec. 31, 2012
Line of Credit
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Oct. 31, 2012
Unsecured Line of Credit [Member]
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Aug. 31, 2012
Unsecured Line of Credit [Member]
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Dec. 31, 2011
Unsecured Line of Credit [Member]
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Dec. 31, 2012
Unsecured Line of Credit [Member]
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Dec. 31, 2011
Unsecured term loan
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Dec. 31, 2012
Unsecured term loan
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Dec. 31, 2012
Federal funds rate
Base rate loans
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Dec. 31, 2012
LIBOR
Base rate loans
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Dec. 31, 2012
LIBOR
Senior unsecured term loan
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Dec. 31, 2012
Minimum
Unsecured Line of Credit [Member]
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Dec. 31, 2012
Maximum
Unsecured Line of Credit [Member]
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| Debt Instrument [Line Items] | ||||||||||||||||||
| Interest rate (as a percent) | 9.00% | 7.125% | 1.10% | 0.10% | ||||||||||||||
| Facility fee (as a percent) | 0.18% | 0.15% | 0.45% | |||||||||||||||
| Current borrowing capacity | $ 2,000,000,000 | |||||||||||||||||
| Variable interest rate | LIBOR | LIBOR | federal funds rate | LIBOR | ||||||||||||||
| Basis spread on variable rate (as a percent) | 1.25% | 1.20% | 0.50% | 1.00% | 1.20% | |||||||||||||
| Maximum borrowing capacity | 2,500,000,000 | |||||||||||||||||
| Maximum available for issuance of letters of credit | 200,000,000 | |||||||||||||||||
| Maximum available for issuance of swingline loans | 200,000,000 | |||||||||||||||||
| Maximum available for issuance of loans in certain alternative currencies | 250,000,000 | |||||||||||||||||
| Percentage of line of credit facility for negotiated rate loans | 50.00% | |||||||||||||||||
| Line of credit facility optional extension period (in years) | 1 year | |||||||||||||||||
| Debt amount, prepaid in full | 200,000,000 | |||||||||||||||||
| Fixed interest rate (as a percent) | 4.00% | |||||||||||||||||
| Amount outstanding | 540,700,000 | 500,000,000 | ||||||||||||||||
| Letters of credit outstanding | 5,900,000 | |||||||||||||||||
| Remaining borrowing capacity | 1,450,000,000 | |||||||||||||||||
| Loss on extinguishment of debt | (37,640,000) | (27,604,000) | (9,791,000) | 2,400,000 | ||||||||||||||
| Principal amount of debt issued | 180,000,000 | 500,000,000 | ||||||||||||||||
| Weighted average maturity on unsecured term loan (in years) | 4 years 6 months | |||||||||||||||||
| Length of tranche | 3 years | |||||||||||||||||
| Length of tranche | 5 years | |||||||||||||||||
| Option to increase the borrowing capacity subject to the satisfaction of certain conditions | $ 900,000,000 | |||||||||||||||||