Long-Term Debt (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended	12 Months Ended				12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Sep. 15, 2011 Series A [Member]	Dec. 31, 2011 Series A [Member] Private Placement [Member]	Dec. 31, 2011 Series B [Member] Private Placement [Member]	Dec. 31, 2011 Master Agreement [Member] years	Dec. 31, 2010 Master Agreement [Member]	Dec. 31, 2011 Master Agreement [Member] Series C [Member]	Dec. 31, 2011 Master Agreement [Member] Series D [Member]	Dec. 31, 2011 Master Agreement [Member] Series E [Member]	Dec. 31, 2011 Loan Agreement [Member]	Dec. 31, 2010 Loan Agreement [Member]	Dec. 31, 2011 LIBOR [Member]	Dec. 31, 2010 LIBOR [Member]	Dec. 31, 2011 Senior Notes [Member]	Dec. 31, 2010 Senior Notes [Member]	Dec. 31, 2011 Senior Notes [Member] Private Placement [Member]
Debt Instrument [Line Items]
Maximum shelf facility capacity							$ 200,000,000
Unsecured senior notes outstanding	250,000,000	250,000,000			100,000,000	100,000,000	150,000,000	50,000,000	25,000,000	25,000,000	100,000,000					100,000,000	200,000,000	200,000,000
Unsecured debt redeemed				100,000,000
Length of issuance period (in years)							3
Long-term debt maturity, period length, maximum (in years)							10
Debt Instrument, issuance date					September 15, 2004	July 15, 2004			December 22, 2006	February 1, 2008	September 15, 2011
Debt instrument, maturity date					Sep. 15, 2011	Jul. 15, 2014			Dec. 22, 2016	Jan. 15, 2015	Sep. 15, 2018
Debt instrument interest rate stated percentage					5.57%	6.08%			5.66%	5.37%	4.50%
Lending commitment amount												50,000,000
Potential maximum increase in lending commitment amount												100,000,000
Availability fees, minimum												0.10%
Availability fees, maximum												0.20%
Letter of credit usage fee, minimum												0.50%
Letter of credit usage fee, maximum												1.00%
90-day LIBOR rate														0.581%	0.30%
Percentage below base rate												1.00%
Interest rate above LIBOR, minimum												0.50%
Interest rate above LIBOR, maximum												1.00%
Additional borrowings												0	0
Interest paid	15,571,000	14,491,000	14,636,000
Maturities of long term debt 2012	1,227,000
Maturities of long term debt 2013	33,000
Maturities of long term debt 2014	100,000,000
Maturities of long term debt 2015	25,000,000
Maturities of long term debt 2016	25,000,000
Maturities of long term debt 2017 and beyond	$ 100,000,000