Fair Value Measurement (Details Textual) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
True-up Liabitlity Discount Rate	3.41%		4.03%
True-up Liability Effect of Change in Discount Rate Increase By One Percent	$ 0.9
True-up Liability Effect of Change in Discount Rate Decrease by One Percent	1.0
George Washington [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair value of true up liability	4.9	4.3	4.5
Midwest [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated fair value of true up liability	$ 6.9	$ 7.2	$ 7.7