Fair Value of Financial Instruments - Carrying Value (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets:
Marketable securities	$ 31,229	$ 23,723
Long term marketable securities	104,632	75,410
Liabilities:
Derivative liabilities		114
Total Carrying Value | Recurring
Assets:
Cash and cash equivalents	61,737	66,398
Marketable securities	31,229	23,723
Long term marketable securities	104,632	75,410
Marketable securities - restricted (SERP)	294	275
Total assets at fair value	197,892	165,806
Liabilities:
Derivative liabilities		114
Quoted Prices in Active Markets (Level 1) | Recurring
Assets:
Cash and cash equivalents	61,737	66,398
Marketable securities	656	656
Total assets at fair value	62,393	67,054
Significant Other Observable Inputs (Level 2) | Recurring
Assets:
Marketable securities	30,573	23,067
Long term marketable securities	104,632	75,410
Marketable securities - restricted (SERP)	294	275
Total assets at fair value	$ 135,499	98,752
Significant Unobservable Inputs (Level 3) | Recurring
Liabilities:
Derivative liabilities		$ 114