Fair Value of Financial Instruments - Carrying Value (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets:
Marketable securities	$ 27,533	$ 23,723
Long term marketable securities	89,163	75,410
Liabilities:
Derivative liabilities	23	114
Total Carrying Value | Recurring
Assets:
Cash and cash equivalents	59,599	66,398
Marketable securities	27,533	23,723
Long term marketable securities	89,163	75,410
Marketable securities - restricted (SERP)	294	275
Total assets at fair value	176,589	165,806
Liabilities:
Derivative liabilities	23	114
Quoted Prices in Active Markets (Level 1) | Recurring
Assets:
Cash and cash equivalents	59,599	66,398
Marketable securities	656	656
Total assets at fair value	60,255	67,054
Significant Other Observable Inputs (Level 2) | Recurring
Assets:
Marketable securities	26,877	23,067
Long term marketable securities	89,163	75,410
Marketable securities - restricted (SERP)	294	275
Total assets at fair value	116,334	98,752
Significant Unobservable Inputs (Level 3) | Recurring
Liabilities:
Derivative liabilities	$ 23	$ 114