Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012 Years	Jun. 30, 2011
Variable Interest Entities (Textuals)
Number of tolling and purchased power agreements with third parties which represent variable interests	2
Number of operating leases with VIEs	1
Operating Lease Agreements with Vie, Remaining Term, In Years	1
Power capacity under capital lease (MW)	236
Capital Lease Agreement With Vie Remaining Term In Years	10
Total payments over remaining terms of the two agreements	$ 283.4
Total capacity and lease payments	$ 29.6	$ 32.1