Risk Management - Risk Concentrations And Interest Rate Risk (Details) (Interest rate swap agreements, USD $)	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2010
Interest rate swap agreements
Risk Management-Risk Concentrations and Interest Rate Risk
Number of agreements held	1
Notional outstanding amount	$ 100,000,000
Remaining maturity term	11 months
Fixed interest rate (as a percent)	2.10%
Amount of loss recognized in the income statement as an increase in interest expense	$ 6,400,000	$ 11,300,000	$ 18,600,000