Fair Value of Financial Instruments - Carrying Value (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Assets:
Marketable securities	$ 26,024	$ 28,692
Long term marketable securities	67,044	55,009
Liabilities:
Derivative liabilities	287	854
Total Carrying Value | Recurring
Assets:
Cash and cash equivalents	54,305	33,498
Marketable securities	26,024	28,692
Long term marketable securities	67,044	55,009
Marketable securities - restricted (SERP)	278	263
Total assets at fair value	147,651	117,462
Liabilities:
Derivative liabilities	287	854
Quoted Prices in Active Markets (Level 1) | Recurring
Assets:
Cash and cash equivalents	54,305	33,498
Marketable securities	654	654
Total assets at fair value	54,959	34,152
Significant Other Observable Inputs (Level 2) | Recurring
Assets:
Marketable securities	25,370	28,038
Long term marketable securities	67,044	55,009
Marketable securities - restricted (SERP)	278	263
Total assets at fair value	92,692	83,310
Significant Unobservable Inputs (Level 3) | Recurring
Liabilities:
Derivative liabilities	$ 287	$ 854