Financial and Other Derivative Instruments (Details Textuals) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Letters of credit that could be used to offset net derivative liabilities	$ 1,000,000	$ 4,000,000
Letters of credit received that could be used to offset net derivative assets	6,000,000	4,000,000
Cash collateral posted, net of cash collateral received	19,000,000	28,000,000
Collateral adjustment on derivative assets	0	9,000,000
Collateral adjustment on derivative liabilities	0	22,000,000
Cash collateral paid	24,000,000	18,000,000
Cash collateral received	5,000,000	$ 3,000,000
Additional collateral, aggregate fair value	508,000,000
Derivative net liability position aggregate fair value	283,000,000
Collateral already posted fair value	0
Derivative net asset position, fair value	247,000,000
Remaining amount of offsets to derivative net liability positions for hard and soft trigger provisions	$ 36,000,000