Fair Value of Financial Instruments - Carrying Value (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets:
Marketable securities	$ 23,723	$ 28,692
Long term marketable securities	75,410	55,009
Liabilities:
Derivative liabilities	114	854
Total Carrying Value | Recurring
Assets:
Cash and cash equivalents	66,398	33,498
Marketable securities	23,723	28,692
Long term marketable securities	75,410	55,009
Marketable securities - restricted (SERP)	275	263
Total assets at fair value	165,806	117,462
Liabilities:
Derivative liabilities	114	854
Quoted Prices in Active Markets (Level 1) | Recurring
Assets:
Cash and cash equivalents	66,398	33,498
Marketable securities	656	654
Total assets at fair value	67,054	34,152
Significant Other Observable Inputs (Level 2) | Recurring
Assets:
Marketable securities	23,067	28,038
Long term marketable securities	75,410	55,009
Marketable securities - restricted (SERP)	275	263
Total assets at fair value	98,752	83,310
Significant Unobservable Inputs (Level 3) | Recurring
Liabilities:
Derivative liabilities	$ 114	$ 854