Fair Value Measurement (Details Textual) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
True-up Liabitlity Discount Rate	3.56%		4.64%
True-up Liability Effect of Change in Discount Rate By One Percent	$ 1.0		$ 0.9
George Washington [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair value of true up liability	4.3	4.3	4.2
Midwest [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair value of true up liability	$ 7.4	$ 7.2	$ 7.4