Fair Value Measurements - Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Fair value on a recurring basis
Industrial revenue development bonds, carrying amount, approximates fair value	$ 7,500	$ 7,500	$ 7,500
Cost Basis | Level 2
Fair value on a recurring basis
Industrial revenue development bonds, carrying amount, approximates fair value	7,500	7,500	7,500
Estimated Fair Value | Level 2
Fair value on a recurring basis
Industrial revenue development bonds, carrying amount, approximates fair value	$ 7,500	$ 7,500	$ 7,500