Long-term debt (Details Textual) (USD $)	3 Months Ended
	Mar. 31, 2013
Long-term Debt (Textual) [Abstract]
Line of credit facility, unused borrowing capacity	$ 887,000,000
Unsecured Debt [Member]
Long-term Debt (Textual) [Abstract]
Line of credit facility, additional unsecured debt borrowing capacity	1,250,000,000.00
Repayment of Debt [Member]
Long-term Debt (Textual) [Abstract]
Reduction in long-term debt reflecting repayments of borrowings, net of discount amortization	17,100,000
Revolving credit agreements, repayments	$ 15,000,000