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Borrowing Arrangements (Details 2) (USD $)
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12 Months Ended | 12 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 1 Months Ended | |||||||||||||||||
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Dec. 31, 2011
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Dec. 31, 2010
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Dec. 31, 2009
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Dec. 31, 2011
NHP
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Dec. 31, 2011
Unsecured revolving credit facilities
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Dec. 31, 2010
Unsecured revolving credit facilities
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Dec. 31, 2011
Base rate loans
Federal funds rate
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Dec. 31, 2011
Base rate loans
LIBOR
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Dec. 31, 2011
Eurocurrency rate loans
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Dec. 31, 2011
Unsecured Revolving Credit Facility
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Dec. 31, 2011
Unsecured Revolving Credit Facility
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Dec. 31, 2011
Unsecured Revolving Credit Facility
Minimum
Y
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Dec. 31, 2011
Unsecured Revolving Credit Facility
Maximum
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Dec. 31, 2011
Senior unsecured term loan
NHP
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Dec. 31, 2011
Senior unsecured term loan
LIBOR
NHP
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Dec. 31, 2011
Unsecured term loan
Y
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Dec. 31, 2011
Unsecured term loan
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Sep. 30, 2010
Unsecured term loan
Bank of America
Y
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Dec. 31, 2011
Unsecured term loan due 2015
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Dec. 31, 2010
Unsecured term loan due 2015
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Dec. 31, 2011
Unsecured term loan due 2017
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Dec. 31, 2010
Unsecured term loan due 2017
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Nov. 30, 2011
3 7/8 % Convertible Senior Notes due 2011 upon maturity
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Dec. 31, 2011
3 7/8 % Convertible Senior Notes due 2011 upon maturity
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Nov. 15, 2011
3 7/8 % Convertible Senior Notes due 2011 upon maturity
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Dec. 31, 2010
3 7/8 % Convertible Senior Notes due 2011 upon maturity
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Mar. 31, 2010
3 7/8 % Convertible Senior Notes due 2011 upon maturity
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| Debt instruments | |||||||||||||||||||||||||||
| Current borrowing capacity | $ 2,000,000,000 | ||||||||||||||||||||||||||
| 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 | ||||||||||||||||||||||||||
| Principal amount of debt issued | 500,000,000 | ||||||||||||||||||||||||||
| Weighted average maturity on unsecured term loan (in years) | 4.5 | ||||||||||||||||||||||||||
| Percentage of line of credit facility for negotiated rate loans | 50.00% | ||||||||||||||||||||||||||
| Variable interest rate | federal funds rate | LIBOR | LIBOR | LIBOR | |||||||||||||||||||||||
| Basis spread on variable rate (as a percent) | 0.50% | 1.00% | 1.50% | 1.25% | 1.25% | ||||||||||||||||||||||
| Length of tranche | 3 | ||||||||||||||||||||||||||
| Length of tranche | 5 | ||||||||||||||||||||||||||
| Percentage of interest rate | 1.10% | ||||||||||||||||||||||||||
| Facility fee (as a percent) | 17.50% | 0.15% | 0.45% | ||||||||||||||||||||||||
| Line of credit facility optional extension period (in years) | 1 | ||||||||||||||||||||||||||
| Amount outstanding | 455,600,000 | 455,600,000 | |||||||||||||||||||||||||
| Letters of credit outstanding | 8,300,000 | 8,300,000 | |||||||||||||||||||||||||
| Loss on extinguishment of debt | 27,604,000 | 9,791,000 | 6,080,000 | (2,400,000) | |||||||||||||||||||||||
| Remaining borrowing capacity | 1,540,000,000 | 1,540,000,000 | |||||||||||||||||||||||||
| Unsecured term loan | 800,000,000 | 200,000,000 | |||||||||||||||||||||||||
| Loan term (in years) | 3 | ||||||||||||||||||||||||||
| Interest rate (as a percent) | 4.00% | 3.875% | 3.875% | 3.875% | 3.875% | 3.875% | |||||||||||||||||||||
| Amount outstanding | 6,180,443,000 | 2,926,954,000 | 652,600,000 | 455,578,000 | 40,000,000 | 500,000,000 | 500,000,000 | 126,875,000 | 0 | 375,000,000 | 0 | 230,000,000 | 0 | 230,000,000 | |||||||||||||
| Option to increase the borrowing capacity subject to the satisfaction of certain conditions | 900,000,000 | 900,000,000 | |||||||||||||||||||||||||
| Principal amount repaid | $ 230,000,000 | ||||||||||||||||||||||||||
| Aggregate shares of common stock issued for the conversion value in excess of the principal amount | 943,714 | ||||||||||||||||||||||||||