Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets:
Marketable securities	$ 46,615	$ 49,211
Long term marketable securities	11,662	8,756
Liabilities:
Derivative liabilities	9,565	12,644
Quoted Prices in Active Markets (Level 1)
Assets:
Cash and cash equivalents	12,252	32,980
Total assets at fair value	12,252	32,980
Significant Other Observable Inputs (Level 2)
Assets:
Marketable securities	46,615	49,211
Long term marketable securities	11,662	8,756
Marketable securities - restricted (SERP)	305	305
Total assets at fair value	58,582	58,272
Significant Unobservable Inputs (Level 3)
Liabilities:
Derivative liabilities	9,565	12,644
Total Carrying Value
Assets:
Cash and cash equivalents	12,252	32,980
Marketable securities	46,615	49,211
Long term marketable securities	11,662	8,756
Marketable securities - restricted (SERP)	305	305
Total assets at fair value	70,834	91,252
Liabilities:
Derivative liabilities	$ 9,565	$ 12,644