Financial Derivatives (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Number of swap agreements	5
Interest rate swaps notional amount	$ 300,000,000
Period of interest rate swap agreements, years	3
Payment of weighted average fixed rate	1.95%
Weighted average fixed rate	0.52%
Net accrued on interest rate swaps	4,900,000	4,800,000
Interest rate derivative liabilities, at fair value	2,600,000	6,500,000
Positive Fair Value [Member]
Derivative [Line Items]
Notional amount of option contracts	4,300,000	29,300,000
Option contracts with net fair value	29,000	35,000
Spot and forward contracts total notional amount	238,600,000	112,700,000
Spot and forward contracts total fair value	2,200,000	4,600,000
Negative Fair Value [Member]
Derivative [Line Items]
Spot and forward contracts total notional amount	128,200,000	68,400,000
Spot and forward contracts total fair value	$ (486,000)	$ (1,900,000)