Commitments And Contingencies (Details) (USD $)	48 Months Ended			12 Months Ended					12 Months Ended
	Dec. 31, 2004	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Minimum [Member]	Dec. 31, 2012 Maximum [Member]	Dec. 31, 2012 Revolving Credit Facility [Member] One-Month London Interbank Offer Rate, Reset Daily [Member]	Dec. 31, 2012 Revolving Credit Facility [Member] One-Month, Two-Month, Three-Month, Six-Month, or Twelve-Month London Interbank Offer Rate [Member]	Dec. 31, 2012 Unsecured Debt [Member] Revolving Credit Facility [Member]	Dec. 31, 2012 Unsecured Debt [Member] Letter of Credit Subfacility [Member]	Dec. 31, 2012 Swing Line Loans [Member] One-Month London Interbank Offer Rate, Reset Daily [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity								$ 125,000,000	$ 40,000,000
Outstanding undrawn amount of letter of credit to insurance carrier		29,250,000
Credit facility, amount outstanding		0
Interest percentage over LIBOR rate						0.875%	0.875%			0.875%
Percentage fee paid for unused portion of line of credit (minimum if quarterly utilization of the facility is less than 20%, maximum if quarterly utilization of the facility is 20% or more)				0.125%	0.10%
Commission fee on amount available to be drawn on letter of credit subfacility									0.875%
Issuance fee as percent of face amount									0.075%
Percentage purchase of outstanding IRB bonds	100.00%
Investment in and outstanding debt for industrial revenue bond		$ 3,200,000	$ 9,733,000