Investments - Investments Carried at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Investments carried at fair value	$ 3,496	$ 3,279
Cash and Equivalents
Debt Securities, Available-for-sale [Line Items]
Investments carried at fair value	1,408	0
Domestic equity funds
Debt Securities, Available-for-sale [Line Items]
Investments carried at fair value	1,675	2,856
International equity funds
Debt Securities, Available-for-sale [Line Items]
Investments carried at fair value	$ 413	$ 423