Fair Value of Financial Instruments - Carrying Value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets:
Marketable securities	$ 32,626	$ 23,723
Long term marketable securities	123,123	75,410
Liabilities:
Derivative liabilities		114
Total Carrying Value | Recurring
Assets:
Cash and cash equivalents	81,924	66,398
Marketable securities	32,626	23,723
Long term marketable securities	123,123	75,410
Marketable securities - restricted (SERP)	313	275
Total assets at fair value	237,986	165,806
Liabilities:
Derivative liabilities		114
Quoted Prices in Active Markets (Level 1) | Recurring
Assets:
Cash and cash equivalents	81,924	66,398
Marketable securities	1,871	656
Long term marketable securities	698
Total assets at fair value	84,493	67,054
Significant Other Observable Inputs (Level 2) | Recurring
Assets:
Marketable securities	30,755	23,067
Long term marketable securities	122,425	75,410
Marketable securities - restricted (SERP)	313	275
Total assets at fair value	$ 153,493	98,752
Significant Unobservable Inputs (Level 3) | Recurring
Liabilities:
Derivative liabilities		$ 114