Long-Term Debt (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Long-Term Debt by Maturity Periods [Abstract]
Amount	$ 367,492	$ 370,344
Callable Amount	345,000	345,000
2013 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	Dec. 31, 2013	Dec. 31, 2013
Amount	119,502	119,400
Weighted Average Rate (in hundredths)	3.87%	3.87%
Callable Amount	100,000	100,000
Weighted Average Rate Callable portion	3.71%	3.71%
2014 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	Dec. 31, 2014	Dec. 31, 2014
Amount	2,610	2,604
Weighted Average Rate (in hundredths)	1.89%	2.00%
Callable Amount	0	0
Weighted Average Rate Callable portion	0.00%	0.00%
2015 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	Dec. 31, 2015	Dec. 31, 2015
Amount	250	0
Weighted Average Rate (in hundredths)	0.00%	0.00%
Callable Amount	0	0
Weighted Average Rate Callable portion	0.00%	0.00%
2016 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	Dec. 31, 2016	Dec. 31, 2016
Amount	70,000	70,000
Weighted Average Rate (in hundredths)	4.21%	4.21%
Callable Amount	70,000	70,000
Weighted Average Rate Callable portion	4.21%	4.21%
2017 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	Dec. 31, 2017	Dec. 31, 2017
Amount	100,000	100,000
Weighted Average Rate (in hundredths)	3.89%	3.89%
Callable Amount	100,000	100,000
Weighted Average Rate Callable portion	3.89%	3.89%
2018 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	Dec. 31, 2018	Dec. 31, 2018
Amount	75,000	75,000
Weighted Average Rate (in hundredths)	3.61%	3.61%
Callable Amount	75,000	75,000
Weighted Average Rate Callable portion	3.61%	3.61%
2021 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	Dec. 31, 2021	Dec. 31, 2021
Amount	130	0
Weighted Average Rate (in hundredths)	4.00%	0.00%
Callable Amount	0	0
Weighted Average Rate Callable portion	0.00%	0.00%
2025 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	Dec. 31, 2025	Dec. 31, 2025
Amount	0	3,340
Weighted Average Rate (in hundredths)	0.00%	2.75%
Callable Amount	$ 0	$ 0
Weighted Average Rate Callable portion	0.00%	0.00%