Commitments and Contingencies Long Term Supply and Capacity Purchase Obligations (Details) (Purchased Coal and Natural Gas Supply And Natural Gas Transportation Contracts [Member], USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Long-term Purchase Commitment [Line Items]
Long Term Purchase Committments Costs Incurred	$ 402,300,000	$ 379,400,000	$ 340,800,000
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2015	206,500,000
2016	161,100,000
2017	134,900,000
2018	107,300,000
2019	103,500,000
Thereafter	$ 922,700,000
Minimum [Member]
Long-term Purchase Commitment [Line Items]
Long Term Purchase Commitments Term In Years	1 year
Maximum [Member]
Long-term Purchase Commitment [Line Items]
Long Term Purchase Commitments Term In Years	27