Fair Value of Financial Instruments (Details) (USD $)			6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Recurring Basis	Jun. 30, 2013 Recurring Basis Level 1	Dec. 31, 2012 Recurring Basis Level 1	Jun. 30, 2013 Recurring Basis Level 2	Dec. 31, 2012 Recurring Basis Level 2	Jun. 30, 2013 Recurring Basis Total	Dec. 31, 2012 Recurring Basis Total
Assets
Money market funds				$ 110,415,000	$ 145,422,000			$ 110,415,000	$ 145,422,000
Derivative financial instruments						37,252,000	13,785,000	37,252,000	13,785,000
Investments in marketable securities	80,501,000	80,341,000				80,501,000	80,341,000	80,501,000	80,341,000
Demand deposits				89,300,000	52,300,000
Liabilities
Derivative financial instruments						60,000		60,000
Transfer of assets from level 1 to level 2			0
Transfer of assets from level 2 to level 1			0
Transfer of liabilities from level 1 to level 2			0
Transfer of liabilities from level 2 to level 1			0
Transfer of assets into level 3			0
Transfer of assets out of level 3			0
Transfer of liabilities into level 3			0
Transfer of liabilities out of level 3			$ 0