Fair Value Measurement - Schedule of Carrying and Fair Value Amounts (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Revolving Credit Facility [Member]
Balance Sheet Grouping
Revolving credit agreement	$ 55,000	$ 55,000	$ 30,000
Level 1 | Other Assets Liabilities, Carrying Value
Balance Sheet Grouping
Held-to-maturity marketable securities	107,266	132,790	127,813
Level 1 | Other Assets, Liabilities, Fair Value
Balance Sheet Grouping
Held-to-maturity marketable securities	106,143	132,002	127,448
Level 3 | Other Assets Liabilities, Carrying Value
Balance Sheet Grouping
2019 Notes	80,000	80,000	120,000
Credit Agreement - term loan	88,750	90,000	93,750
Level 3 | Other Assets Liabilities, Carrying Value | Revolving Credit Facility [Member]
Balance Sheet Grouping
Revolving credit agreement	55,000	55,000	30,000
Level 3 | Other Assets, Liabilities, Fair Value
Balance Sheet Grouping
2019 Notes	83,086	82,190	125,930
Credit Agreement - term loan	88,803	89,871	92,986
Level 3 | Other Assets, Liabilities, Fair Value | Revolving Credit Facility [Member]
Balance Sheet Grouping
Revolving credit agreement	$ 54,858	$ 55,054	$ 29,520