Financial and Other Derivative Instruments (Details Textuals) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Letters of credit that could be used to offset net derivative liabilities	$ 0	$ 7,000,000
Letters of credit received that could be used to offset net derivative assets	1,000,000	5,000,000
Cash collateral posted, net of cash collateral received	85,000,000	61,000,000
Derivative Asset, Collateral, Obligation to Return Cash, Offset	1,000,000	0
Collateral adjustment	46,000,000	19,000,000
Cash collateral received	3,000,000	2,000,000
Cash collateral paid	43,000,000	44,000,000
Additional collateral, aggregate fair value	416,000,000
Derivative net liability position aggregate fair value	726,000,000
Collateral already posted fair value	4,000,000
Derivative net asset position, fair value	630,000,000
Remaining amount of offsets to derivative net liability positions for hard and soft trigger provisions	$ 92,000,000