Financial Derivatives (Details) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Number of swap agreements	5
Financial institution notional amount	$ 300,000,000
Period of interest rate swap agreements, years	3
Payment of weighted average fixed rate	1.95%
Proceed from weighted average fixed rate	0.25%
Net accrued on interest rate swaps	2,500,000
Interest rate derivative liabilities, at fair value	5,300,000	6,500,000
Positive Fair Value [Member]
Notional amount of option contract	37,500,000	29,300,000
Option contract with net positive fair value	84,000	35,000
Spot and forward contract total notional amount	172,500,000	112,700,000
Spot and forward contract total fair value	1,700,000	4,600,000
Negative Fair Value [Member]
Spot and forward contract total notional amount	113,700,000	68,400,000
Spot and forward contract total fair value	$ (391,000)	$ (1,900,000)