Derivative financial instruments (Details) (Fixed rate long-term borrowings [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Fixed rate long-term borrowings [Member]
Information about interest rate swap agreements
Notional amount	$ 900,000	$ 900,000
Average maturity (in years)	5.9	6.4
Weighted-average rate, fixed	6.07%	6.07%
Weighted-average rate, variable	1.79%	1.84%