Short-Term and Long-Term Debt Narrative (Details) (USD $)					12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Jul. 22, 2010	Dec. 31, 2009	Dec. 31, 2012 Minimum [Member]	Dec. 31, 2012 Maximum [Member]	Dec. 31, 2012 Line of Credit [Member]	Dec. 31, 2012 Line of Credit [Member] Base Rate, Libor [Member]	Dec. 31, 2012 Line of Credit [Member] Base Rate, Federal Funds [Member]	Dec. 31, 2012 Line of Credit [Member] Base Rate [Member] Minimum [Member]	Dec. 31, 2012 Line of Credit [Member] Base Rate [Member] Maximum [Member]	Dec. 31, 2012 Line of Credit [Member] Libor [Member] Minimum [Member]	Dec. 31, 2012 Line of Credit [Member] Libor [Member] Maximum [Member]	Dec. 31, 2012 Letter of Credit [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate	5.09%
Senior unsecured notes	$ 30,000,000	$ 30,000,000	$ 30,000,000
Revolving credit facility, current borrowing capacity	140,000,000			159,000,000
Number of principal payments	5
Anniversary when principal payments begin	6
Letter of credit sublimit	100,000,000
Revolving credit facility, maximum borrowing capacity	160,000,000
Unused commitments under credit facility	66,913,000
Committed under credit facility	11,015,000						73,087,000
Revolving credit facility borrowings	24,200,000	20,000,000
Letters of credit outstanding, amount	50,395,000													48,887,000
Outstanding borrowings interest rate margin adjustment range										0.00%	0.25%	1.25%	1.75%
Debt instrument, variable rate, base rate calculation								1.50%	0.50%
Line of credit facility, commitment fee percentage					0.20%	0.30%
Term loans, weighted average interest rate	1.99%	2.37%
Long-term Debt, Fiscal Year Maturity [Abstract]
2013	6,992,000
2014	24,754,000
2015	33,000
2016	6,000,000
2017	$ 6,000,000