Fair Value of Financial Instruments - Carrying Value (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Assets:
Marketable securities	$ 24,058	$ 28,038
Long term marketable securities	67,809	55,009
Liabilities:
Derivative liabilities	412	854
Total Carrying Value | Recurring
Assets:
Cash and cash equivalents	36,156	34,152
Marketable securities	24,058	28,038
Long term marketable securities	67,809	55,009
Marketable securities - restricted (SERP)	266	263
Total assets at fair value	128,289	117,462
Liabilities:
Derivative liabilities	412	854
Quoted Prices in Active Markets (Level 1) | Recurring
Assets:
Cash and cash equivalents	36,156	34,152
Total assets at fair value	36,156	34,152
Significant Other Observable Inputs (Level 2) | Recurring
Assets:
Marketable securities	24,058	28,038
Long term marketable securities	67,809	55,009
Marketable securities - restricted (SERP)	266	263
Total assets at fair value	92,133	83,310
Significant Unobservable Inputs (Level 3) | Recurring
Liabilities:
Derivative liabilities	$ 412	$ 854