Fair Value Measurements - Assets Measured at Fair Value on Recurring Basis (Details) - Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	$ 17,627	$ 16,281
Marketable Equity Security [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	12	11
Equity Mutual Fund [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	8,840	3,764
U.S. Treasury Mutual Fund [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	8,775	12,506
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	8,787	12,517
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Marketable Equity Security [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	12	11
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Equity Mutual Fund [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Treasury Mutual Fund [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	8,775	12,506
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	8,840	3,764
Significant Other Observable Inputs (Level 2) [Member] | Marketable Equity Security [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | Equity Mutual Fund [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	8,840	3,764
Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury Mutual Fund [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	0	0
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Marketable Equity Security [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | Equity Mutual Fund [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	0	0
Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury Mutual Fund [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	$ 0	$ 0