Disclosures About Fair Value - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets
Available-for-sale equity securities	$ 1,259
Available-for-sale equity securities		$ 1,218
Debt securities: corporate bonds	654	684
Carrying Value		1,902
Fair Value Measurements on a Recurring Basis
Assets
Available-for-sale equity securities	1,260
Available-for-sale equity securities		1,219
Debt securities: corporate bonds	653	683
Carrying Value	1,913	1,902
Fair Value Measurements on a Recurring Basis | Quoted Prices In Active Markets for Identical Assets (Level 1)
Assets
Available-for-sale equity securities	981
Available-for-sale equity securities		975
Debt securities: corporate bonds	653	683
Carrying Value	1,634	1,658
Fair Value Measurements on a Recurring Basis | Significant Other Observable Inputs (Level 2)
Assets
Available-for-sale equity securities	279
Available-for-sale equity securities		244
Debt securities: corporate bonds
Carrying Value	279	244
Fair Value Measurements on a Recurring Basis | Significant Unobservable Inputs (Level 3)
Assets
Available-for-sale equity securities
Available-for-sale equity securities
Debt securities: corporate bonds
Carrying Value