Long-Term Debt Schedule of Debt (Details) (USD $) In Thousands, unless otherwise specified				9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Sep. 30, 2014 Industrial Development Debt Payable Through 2022, 0.30%	Dec. 31, 2013 Industrial Development Debt Payable Through 2022, 0.30%	Sep. 30, 2014 2006 Senior Notes Payable Through 2021, 5.41%	Dec. 31, 2013 2006 Senior Notes Payable Through 2021, 5.41%	Sep. 30, 2014 2010 Senior Notes Payable Through 2021, 4.00%	Dec. 31, 2013 2010 Senior Notes Payable Through 2021, 4.00%
Debt Instrument [Line Items]
Senior Notes	$ 153,300		$ 160,000			$ 53,334		$ 100,000
Senior notes payable							53,334		100,000
Senior revolving credit facility maturing in 2019	124,000	110,000
Total	277,334	267,334		0	4,000
Amounts due within one year	6,667	6,667
Long-term debt	$ 270,667	$ 260,667
Debt instruments maturity date				2022	2022	2021	2021	2021	2021
Debt instrument, stated interest rate percentage				0.30%	0.30%	5.41%	5.41%	4.00%	4.00%