Fair Value Measurements - Contractual Maturities of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Maturing in one year or less	$ 928	$ 916
Maturing in second year through fifth year	20,688	15,948
Maturing in sixth year through tenth year	18,515	22,851
Maturing after ten years	34,573	33,506
Total debt securities	$ 74,704	$ 73,221

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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