Short-Term and Long-Term Debt Narrative (Details) (USD $) In Thousands, unless otherwise specified				12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Jul. 22, 2010	Dec. 31, 2013 Minimum [Member]	Dec. 31, 2013 Maximum [Member]	Dec. 31, 2013 Line of Credit [Member]	Dec. 31, 2013 Line of Credit [Member] Base Rate, Libor [Member]	Dec. 31, 2013 Line of Credit [Member] Base Rate, Federal Funds [Member]	Dec. 31, 2013 Line of Credit [Member] LIBOR [Member] Minimum [Member]	Dec. 31, 2013 Line of Credit [Member] LIBOR [Member] Maximum [Member]	Dec. 31, 2013 Letter of Credit [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate	5.09%
Revolving credit facility, current borrowing capacity	$ 175,000	$ 140,000
Letter of credit sublimit	100,000	100,000
Swing line commitment	15,000
Revolving credit facility, maximum borrowing capacity	210,000
Senior unsecured notes	30,000	30,000	30,000
Number of principal payments	5
Anniversary when principal payments begin	6
Unused commitments under credit facility	101,096
Committed under credit facility	11,217					73,904
Revolving credit facility borrowings	20,000	24,200
Letters of credit outstanding, amount	54,909										53,904
Outstanding borrowings interest rate margin adjustment range									0.75%	1.50%
Debt instrument, variable rate, base rate calculation							1.50%	0.50%
Line of credit facility, commitment fee percentage				0.10%	0.25%
Term loans, weighted average interest rate	2.04%	1.99%
Long-term Debt, Fiscal Year Maturity [Abstract]
2014	8,434
2015	34
2016	6,000
2017	6,000
2018	$ 26,000