Fair Value of Financial Instruments (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Assets
Derivative financial instruments	$ 27,461,000	$ 13,785,000	$ (305,000)	$ 2,275,000
Investments in marketable securities	80,473,000	80,341,000
Recurring Basis | Level 1
Assets
Money market funds	145,400,000	145,422,000
Demand deposits	73,200,000	52,300,000
Recurring Basis | Level 2
Assets
Derivative financial instruments	27,461,000	13,785,000
Investments in marketable securities	80,473,000	80,341,000
Recurring Basis | Total
Assets
Money market funds	145,400,000	145,422,000
Derivative financial instruments	274,461,000	13,785,000
Investments in marketable securities	$ 80,473,000	$ 80,341,000