Fair Value Measurement - Carrying and Fair Value Amounts (Details) - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Revolving Credit Facility [Member]
Balance Sheet Grouping
Revolving credit agreement		$ 30,000	$ 30,000
Level 1 | Other Assets, Liabilites, Fair Value
Balance Sheet Grouping
Held-to-maturity marketable securities		107,381	127,365	$ 77,678
Level 1 | Other Assets Liabilities, Carrying Value
Balance Sheet Grouping
Held-to-maturity marketable securities		107,811	127,779	77,612
Level 3 | Other Assets, Liabilites, Fair Value
Balance Sheet Grouping
Senior notes payable	[1]	123,371	124,654	167,210
Credit Agreement loan	[1]	92,046	93,991	97,409
Level 3 | Other Assets, Liabilites, Fair Value | Revolving Credit Facility [Member]
Balance Sheet Grouping
Revolving credit agreement	[1]	29,672	29,452	0
Level 3 | Other Assets Liabilities, Carrying Value
Balance Sheet Grouping
Senior notes payable	[1]	120,000	120,000	160,000
Credit Agreement loan	[1]	92,500	95,000	97,500
Level 3 | Other Assets Liabilities, Carrying Value | Revolving Credit Facility [Member]
Balance Sheet Grouping
Revolving credit agreement	[1]	$ 30,000	$ 30,000	$ 0

[1] 1See Note 10 for definitions of 2019 Notes and Credit Agreement.