Long-Term Debt (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Long-Term Debt by Maturity Periods [Abstract]
Amount	$ 370,344	$ 369,874
Callable Amount	345,000	346,994
2011 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	2011	2011
Amount	0	1,994
Weighted Average Rate (in hundredths)	0.00%	4.73%
Callable Amount	0	1,994
Weighted Average Rate (in hundredths)	0.00%	4.73%
2013 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	2013	2013
Amount	119,400	119,400
Weighted Average Rate (in hundredths)	3.87%	3.87%
Callable Amount	100,000	100,000
Weighted Average Rate (in hundredths)	3.71%	3.71%
2014 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	2014	2014
Amount	2,604	0
Weighted Average Rate (in hundredths)	2.00%	0.00%
Callable Amount	0	0
Weighted Average Rate (in hundredths)		0.00%
2016 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	2016	2016
Amount	70,000	70,000
Weighted Average Rate (in hundredths)	4.21%	4.21%
Callable Amount	70,000	70,000
Weighted Average Rate (in hundredths)	4.21%	4.21%
2017 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	2017	2017
Amount	100,000	100,000
Weighted Average Rate (in hundredths)	3.89%	3.89%
Callable Amount	100,000	100,000
Weighted Average Rate (in hundredths)	3.89%	3.89%
2018 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	2018	2018
Amount	75,000	75,000
Weighted Average Rate (in hundredths)	3.61%	3.61%
Callable Amount	75,000	75,000
Weighted Average Rate (in hundredths)	3.61%	3.61%
2025 [Member]
Long-Term Debt by Maturity Periods [Abstract]
Maturity year	2025	2025
Amount	3,340	3,480
Weighted Average Rate (in hundredths)	2.75%	2.75%
Callable Amount	$ 0	$ 0
Weighted Average Rate (in hundredths)	0.00%	0.00%