Fair Value of Financial Instruments - Carrying Value (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Assets:
Marketable securities	$ 25,427	$ 28,692	$ 39,678
Long term marketable securities	68,790	55,009	27,315
Liabilities:
Derivative liabilities	535	854	2,691
Total Carrying Value | Recurring
Assets:
Cash and cash equivalents	19,830	33,498	25,158
Marketable securities	25,427	28,692	39,678
Long term marketable securities	68,790	55,009	27,315
Marketable securities - restricted (SERP)	255	263	267
Total assets at fair value	114,302	117,462	92,418
Liabilities:
Derivative liabilities	535	854	2,691
Quoted Prices in Active Markets (Level 1) | Recurring
Assets:
Cash and cash equivalents	19,830	33,498	25,158
Marketable securities	654	654	652
Total assets at fair value	20,484	34,152	25,810
Significant Other Observable Inputs (Level 2) | Recurring
Assets:
Marketable securities	24,773	28,038	39,026
Long term marketable securities	68,790	55,009	27,315
Marketable securities - restricted (SERP)	255	263	267
Total assets at fair value	93,818	83,310	66,608
Significant Unobservable Inputs (Level 3) | Recurring
Liabilities:
Derivative liabilities	$ 535	$ 854	$ 2,691