Long-term debt (Details Textual) (USD $)	6 Months Ended
	Jun. 24, 2012
Long-term Debt (Textual) [Abstract]
Long-term debt, maturity date	2012-04
Minimum [Member]
Long-term Debt (Textual) [Abstract]
Long-term debt, interest rate	6.375%
Unsecured notes bearing fixed rate interest at 6.375% repaid April 2012 [Member]
Long-term Debt (Textual) [Abstract]
Long-term debt, interest rate	6.375%
Unsecured notes bearing fixed rate interest at 8.75% due November 2014 [Member]
Long-term Debt (Textual) [Abstract]
Long-term debt, interest rate	8.75%
Unsecured notes bearing fixed rate interest at 10% due June 2015 [Member]
Long-term Debt (Textual) [Abstract]
Long-term debt, interest rate	10.00%
Unsecured notes bearing fixed rate interest at 6.375% due September 2015 [Member]
Long-term Debt (Textual) [Abstract]
Long-term debt, interest rate	6.375%
Unsecured notes bearing fixed rate interest at 10% due April 2016 [Member]
Long-term Debt (Textual) [Abstract]
Long-term debt, interest rate	10.00%
Unsecured notes bearing fixed rate interest at 9.375% due November 2017 [Member]
Long-term Debt (Textual) [Abstract]
Long-term debt, interest rate	9.375%
Unsecured notes bearing fixed rate interest at 7.125% due September 2018 [Member]
Long-term Debt (Textual) [Abstract]
Long-term debt, interest rate	7.125%
Unsecured Debt [Member]
Long-term Debt (Textual) [Abstract]
Line of credit facility, additional unsecured debt borrowing capacity	1,000,000,000
Revolving Credit Facility [Member]
Long-term Debt (Textual) [Abstract]
Line of credit facility, unused borrowing capacity	665,000,000
Repayment of Debt [Member]
Long-term Debt (Textual) [Abstract]
Reduction in long-term debt reflecting repayments of borrowings, net of discount amortization	97,600,000
Revolving credit agreements, repayments	102,000,000