Financial and Other Derivative Instruments (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Letters of credit that could be used to offset net derivative liabilities	$ 4,000,000	$ 2,000,000
Letters of credit that could be used to offset net derivative assets	4,000,000	2,000,000
Cash collateral posted, net of cash collateral received	28,000,000	34,000,000
Collateral adjustment on derivative assets	9,000,000	7,000,000
Collateral adjustment on derivative liabilities	22,000,000	28,000,000
Cash collateral paid	18,000,000	18,000,000
Cash collateral received	3,000,000	$ 5,000,000
Additional collateral, aggregate fair value	456,000,000
Derivative net liability position aggregate fair value	535,000,000
Collateral already posted fair value	0
Derivative, net asset position, fair value	468,000,000
Remaining amount of offsets to derivative net liability positions for hard and soft trigger provisions	$ 67,000,000