Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 73,633	$ 73,470
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax	(25)	(2)
Available-for-sale Securities	73,608	73,468
Municipal Bonds and Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Amortized Cost Basis	13,320	43,490
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax	(1)	0
Available-for-sale Securities	13,319	43,490
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Amortized Cost Basis	49,432	1,673
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax	(24)	(2)
Available-for-sale Securities	49,408	1,671
Financial Institution Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities, Amortized Cost Basis	10,881	28,307
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax	0	0
Available-for-sale Securities	$ 10,881	$ 28,307