Summary Of Significant Accounting Policies (Schedule Of Fair Values Of The Net Assets (Liabilities)) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011
Derivative Counter-Party [Line Items]
Total assets (liabilities)	$ 33,800
Bank Of Montreal [Member]
Derivative Counter-Party [Line Items]
Total assets (liabilities)	18,700
Bank Of America, N.A. [Member]
Derivative Counter-Party [Line Items]
Total assets (liabilities)	6,500
BNP Paribas [Member]
Derivative Counter-Party [Line Items]
Total assets (liabilities)	6,100
Comerica Bank [Member]
Derivative Counter-Party [Line Items]
Total assets (liabilities)	1,400
BBVA Compass Bank [Member]
Derivative Counter-Party [Line Items]
Total assets (liabilities)	1,000
BP Corporation [Member]
Derivative Counter-Party [Line Items]
Total assets (liabilities)	200
Credit Agricole Corporate And Investment Bank, London Branch [Member]
Derivative Counter-Party [Line Items]
Total assets (liabilities)	100
Macquarie Bank [Member]
Derivative Counter-Party [Line Items]
Total assets (liabilities)	$ (200)