Fair Value Measurements - Contractual Maturities of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Maturing in one year or less	$ 1,403	$ 973
Maturing in second year through fifth year	23,879	28,671
Maturing in sixth year through tenth year	19,346	21,651
Maturing after ten years	25,756	16,435
Total debt securities	$ 70,384	$ 67,730