Investments - Investments Carried at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 3,336	$ 3,279
Domestic equity funds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	2,933	2,856
International equity funds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 403	$ 423