Other Transactions (Sales Agreement and Development Activities) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Accounts receivable from affiliates	$ 28.1	$ 4.0
Assets held for sale	1.5	31.4
Proceeds from the sale of assets	35.9	3.4	19.9
Coyote Creek
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Lignite sales agreements, expected annual delivery	2,500,000
Lignite sales agreements, renewal term	5 years
Accounts receivable from affiliates	24.8
NACoal
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Reimbursement for previously recognized costs for pre-development activities			7.6
Reimbursement of previously recognized costs for pre-development activities	3.5
Assets held for sale		31.4
Long Lived Assets Held-for-sale, Proceeds from Sale	31.2
Long Lived Assets Held-for-sale, Gain (Loss) on Sale	3.3
Gain on sale of land	3.5
Great American Energy | NACoal
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Proceeds from the sale of assets			$ 11.2