Financial Derivatives (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2010
Derivative [Line Items]
Number of swap agreements	5	5
Interest rate swaps notional amount	$ 300,000,000	$ 300,000,000
Period of interest rate swap agreements, years	3	3
Payment of weighted average fixed rate	1.95%	1.95%
Proceed from weighted average fixed rate	0.32%	0.32%
Net accrued on interest rate swaps	3,700,000	3,700,000
Interest rate derivative liabilities, at fair value	3,900,000	6,500,000
Positive Fair Value [Member]
Derivative [Line Items]
Notional amount of option contracts		29,300,000
Option contracts with net fair value		35,000
Spot and forward contracts total notional amount	121,800,000	112,700,000
Spot and forward contracts total fair value	1,200,000	4,600,000
Negative Fair Value [Member]
Derivative [Line Items]
Notional amount of option contracts	36,400,000
Option contracts with net fair value	(340,000)
Spot and forward contracts total notional amount	124,100,000	68,400,000
Spot and forward contracts total fair value	$ (3,900,000)	$ (1,900,000)