Long-term debt (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Debt Disclosure [Abstract]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	$ 23,132
Long-term Debt, Maturities, Repayments of Principal in Year Two	32,005
Long-term Debt, Maturities, Repayments of Principal in Year Three	44,973
Long-term Debt, Maturities, Repayments of Principal in Year Four	107,006
Long-term Debt, Maturities, Repayments of Principal in Year Five	274,075
Long-term Debt, Maturities, Repayments of Principal after Year Five	1,453,131
Long-term Debt, Gross	1,934,322
Debt Instrument, Unamortized Discount	(8,258)
Long-term Debt	$ 1,926,064	$ 1,529,798

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Including current and noncurrent portions, aggregate carrying amount of long-term borrowings as of the balance sheet date before deducting unamortized discount or premiums (if any). May include notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, which had initial maturities beyond one year or beyond the normal operating cycle, if longer.

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The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

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Including current and noncurrent portions, aggregate carrying amount of long-term borrowings as of the balance sheet date. May include notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, which had initial maturities beyond one year or beyond the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any.

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Amount of long-term debt maturities after year five following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed of determinable prices and dates.

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Amount of long-term debt maturing within the next twelve months following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates.

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Amount of long-term debt maturing in year five following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed of determinable prices and dates.

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Amount of long-term debt maturing in year four following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed of determinable prices and dates.

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Amount of long-term debt maturing in year three following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed of determinable prices and dates.

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Amount of long-term debt maturing in year two following the date of the latest balance sheet presented in the financial statements, which may include maturities of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates.

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