Other Events and Transactions (Sales Agreement and Development Activities) (Details) (USD $)				12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012 NACoal	Dec. 31, 2012 Coyote Creek T	Mar. 31, 2013 Coyote Creek
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Long lived assets held-for-sale, proceeds from sale			$ 20,200,000
Lignite sales agreements, expected annual delivery				2,500,000
Lignite sales agreements, renewal term				5 years
Accounts receivable from affiliates	$ 27,642,000	$ 28,144,000		$ 24,800,000	$ 25,200,000