Investments - Available for Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Maturing in one year or less	$ 179,272
Maturing after one year through two years	47,407
Total debt investments	226,679
Total investments	227,965	$ 323,769
Common Stock
Schedule of Investments [Line Items]
Total investments	$ 1,286