Acquisitions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Jan. 30, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition	Jan. 30, 2014
Business Acquisition, Transaction Costs		$ 27.0
Escrow Deposit Disbursements Related to Property Acquisition			$ 5.4
Lake Benton [Member]
Business Acquisition [Line Items]
Business Acquisition, Name of Acquired Entity	Lake Benton
Generating Capacity (MW)		104
Storm Lake [Member]
Business Acquisition [Line Items]
Business Acquisition, Name of Acquired Entity	Storm Lake
Generating Capacity (MW)		77
Condon [Member]
Business Acquisition [Line Items]
Business Acquisition, Name of Acquired Entity	Condon
Generating Capacity (MW)		50