Long-Term Debt (Narrative) (Details) (USD $) In Thousands, unless otherwise specified			6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Minimum [Member]	Jun. 30, 2012 Maximum [Member]	Jun. 30, 2012 Line of credit [Member]	Jun. 30, 2012 Line of credit [Member] Base rate, LIBOR [Member]	Jun. 30, 2012 Line of credit [Member] Base rate, federal funds [Member]	Jun. 30, 2012 Line of credit [Member] Base rate [Member] Minimum [Member]	Jun. 30, 2012 Line of credit [Member] Base rate [Member] Maximum [Member]	Jun. 30, 2012 Line of credit [Member] LIBOR [Member] Minimum [Member]	Jun. 30, 2012 Line of credit [Member] LIBOR [Member] Maximum [Member]	Jun. 30, 2012 Letter of credit [Member]
Debt Instrument [Line Items]
Revolving credit facility, current borrowing Capacity	$ 140,000
Letter of credit, sublimit	100,000
Revolving credit facility, maximum borrowing Capacity	160,000
Unused commitments under credit facility	66,913
Committed under credit facility					73,087
Revolving credit facility borrowings	28,400	20,000
Letters of credit issued												44,687
Outstanding borrowings interest rate, 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%
Senior unsecured notes	$ 30,000	$ 30,000
Senior unsecured notes, interest rate	5.09%