Fair Value of Financial Instruments - Carrying Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Marketable securities	$ 39,736	$ 23,723
Liabilities:
Derivative liabilities		114
Total Carrying Value | Recurring
Assets:
Cash and cash equivalents	100,304	66,398
Marketable securities	39,736	23,723
Corporate debt securities	132,477	74,343
Government debt securities	1,161	1,067
Marketable securities - restricted (SERP)	335	275
Total assets at fair value	274,013	165,806
Liabilities:
Derivative liabilities		114
Quoted Prices in Active Markets (Level 1) | Recurring
Assets:
Cash and cash equivalents	100,304	66,398
Marketable securities	2,118	656
Corporate debt securities	448	1,717
Total assets at fair value	102,870	68,771
Significant Other Observable Inputs (Level 2) | Recurring
Assets:
Marketable securities	37,618	23,067
Corporate debt securities	132,029	72,626
Government debt securities	1,161	1,067
Marketable securities - restricted (SERP)	335	275
Total assets at fair value	$ 171,143	97,035
Significant Unobservable Inputs (Level 3) | Recurring
Liabilities:
Derivative liabilities		$ 114