Fair Value Measurement (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]

The following tables summarize significant assets and liabilities measured at fair value in the consolidated balance sheets on a recurring basis for each of the fair value levels (in thousands):

	Fair Value Measurement at Reporting Date Using
December 31, 2017	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	37,284	$	—	$	—	$	37,284
Commercial paper	9,967		—		—		9,967
Total assets	$	47,251	$	—	$	—	$	47,251

	Fair Value Measurement at Reporting Date Using
December 31, 2016	Level 1		Level 2		Level 3		Total
Cash equivalents
Money market funds	$	10,057	$	—	$	—	$	10,057
Total assets	$	10,057	$	—	$	—	$	10,057

Fair Value, by Balance Sheet Grouping [Table Text Block]

The carrying values and estimated fair values of our financial instruments that are not required to be recorded at fair value in the consolidated balance sheets are as follows (in thousands): 

December 31,		2017				2016
	Fair Value Hierarchy	Carrying Value		Fair Value		Carrying Value		Fair Value
Assets:
Held-to-maturity marketable securities	Level 1	$	132,790	$	132,002	$	127,779	$	127,365
Liabilities (including current maturities):
Senior notes payable1	Level 3	$	80,000	$	82,190	$	120,000	$	124,654
Credit Agreement term loan1	Level 3	$	90,000	$	89,871	$	95,000	$	93,991
Credit Agreement - revolving credit facility, 2016 draw1	Level 3	$	30,000	$	30,105	$	30,000	$	29,452
Credit Agreement - revolving credit facility, 2017 draw1	Level 3	$	25,000	$	24,949	$	—	$	—

¹The fair values of the senior notes payable and Credit Agreement (defined in Note 11) loan are based on borrowing rates available to us for long-term loans with similar terms, average maturities, and credit risk.