Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	$ 15,590	$ 15,669
Liabilities, fair value disclosure, non-recurring	0	0
Equity Mutual Fund (Marketable Securities) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	3,797	3,803
U.S. Treasury Mutual Fund (Marketable Securities) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	11,793
U.S. Treasury Securities (Cash and Cash Equivalents) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets		11,866
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	3,797	15,669
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Equity Mutual Fund (Marketable Securities) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	3,797	3,803
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Treasury Mutual Fund (Marketable Securities) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Treasury Securities (Cash and Cash Equivalents) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets		11,866
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	11,793	0
Significant Other Observable Inputs (Level 2) [Member] | Equity Mutual Fund (Marketable Securities) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	0	0
Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury Mutual Fund (Marketable Securities) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	11,793
Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury Securities (Cash and Cash Equivalents) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets		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] | Equity Mutual Fund (Marketable Securities) [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 (Marketable Securities) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets	$ 0
Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury Securities (Cash and Cash Equivalents) [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Fair value assets		$ 0