Acquisition (Details)	3 Months Ended
	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2014 USD ($)	Oct. 26, 2016 MW	Oct. 09, 2016 USD ($)	Sep. 30, 2016 USD ($)	Apr. 01, 2016 USD ($) MW	Feb. 05, 2016 USD ($) MW	Dec. 31, 2015 USD ($)	Oct. 01, 2015 USD ($)
Business Acquisition [Line Items]
Current assets held for sale						$ 452,000,000			$ 0
Granite Ridge Energy Center [Member]
Business Acquisition [Line Items]
Business combination, recognized identifiable assets acquired and liabilities assumed, net								$ 500,000,000
Power generation capacity | MW								745
Summer Peaking Capacity | MW								695
Crane Champion Holdco, LLC [Member]
Business Acquisition [Line Items]
Ownership percentage of acquiree										75.00%
EDF Trading North America, LLC [Member]
Business Acquisition [Line Items]
Ownership percentage of acquiree										25.00%
Champion Energy Marketing, LLC [Member]
Business Acquisition [Line Items]
Business combination, recognized identifiable assets acquired and liabilities assumed, net										$ 240,000,000
South Point Energy Center [Member]
Business Acquisition [Line Items]
Proceeds from Sale of Productive Assets		$ 76,000,000
Net present value of transmission capacity payment obligations							$ 112,000,000
Remaining tribal lease costs							9,000,000
Near-term repairs, maintenance and capital improvements to restore the power plant to full capacity							$ 21,000,000
Summer Peaking Capacity | MW							504
Osprey Energy Center [Member]
Business Acquisition [Line Items]
Proceeds from Sale of Productive Assets			$ 166,000,000
Osprey Energy Center Agreement Term			27 months
Subsequent Event [Member] | NAES [Member]
Business Acquisition [Line Items]
Business combination, recognized identifiable assets acquired and liabilities assumed, net					$ 800,000,000
Working Capital Adjustment to Sale Price	$ 100,000,000
Expected recovery through collateral synergies					$ 200,000,000
Number of States in which Entity Operates					18
Subsequent Event [Member] | Mankato [Member]
Business Acquisition [Line Items]
Proceeds from Sale of Productive Assets	396,000,000
Gain (Loss) on Sale of Assets and Asset Impairment Charges	$ 160,000,000
Power generation capacity | MW				375
Expansion generation capacity | MW				345
Other Current Assets [Member]
Business Acquisition [Line Items]
Current assets held for sale						12,000,000
Property, Plant and Equipment [Member]
Business Acquisition [Line Items]
Current assets held for sale						401,000,000
Other Assets [Member]
Business Acquisition [Line Items]
Current assets held for sale						$ 39,000,000