Fair Value of Financial Instruments (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Assets
Derivative financial instruments	$ 13,785,000	$ 2,275,000
Investments in marketable securities	80,341,000	75,228,000
Recurring Basis | Level 1
Assets
Money market funds	145,422,000	238,812,000
Demand deposits	51,900,000	119,000,000
Recurring Basis | Level 2
Assets
Commercial paper and short term marketable securities		45,897,000
Derivative financial instruments	13,785,000	2,275,000
Investments in marketable securities	80,341,000	75,228,000
Recurring Basis | Total
Assets
Money market funds	145,422,000	238,812,000
Commercial paper and short term marketable securities		45,897,000
Derivative financial instruments	13,785,000	2,275,000
Investments in marketable securities	$ 80,341,000	$ 75,228,000