Investment Securities Table (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Securities, Current	$ 14,180	$ 10,061
Available-for-sale Securities, Debt Securities, Noncurrent	9,896	14,579
Available-for-sale Debt Securities, Amortized Cost Basis	24,076	24,640
Available-for-sale Debt Securities, Gross Unrealized Loss	0	0
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	14,180	10,061
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	9,896	14,579
Available-for-sale Securities, Debt Securities	24,076	24,640
Available-for-sale Debt Security Current [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	0	0
Available-for-sale Debt Security Noncurrent [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 0	$ 0

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Fair value of available-for-sale debt securities maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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