Fair Value Measurements - Contractual Maturities of Available for Sale Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Maturing in one year or less	$ 973	$ 774
Maturing in second year through fifth year	28,671	13,852
Maturing in sixth year through tenth year	21,651	36,273
Maturing after ten years	16,435	18,597
Total debt securities	$ 67,730	$ 69,496	$ 11,799