Financial and Other Derivative Instruments (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Letters of credit that could be used to offset net derivative liabilities	$ 4,000,000	$ 4,000,000
Letters of credit that could be used to offset net derivative assets	8,000,000	4,000,000
Cash collateral received, net of cash collateral posted	13,000,000
Cash collateral posted, net of cash collateral received		28,000,000
Collateral adjustment on derivative assets	17,000,000	9,000,000
Collateral adjustment on derivative liabilities	0	22,000,000
Cash collateral paid	10,000,000	18,000,000
Cash collateral received	6,000,000	$ 3,000,000
Additional collateral, aggregate fair value	638,000,000
Derivative net liability position aggregate fair value	541,000,000
Collateral already posted fair value	0
Derivative, net asset position, fair value	495,000,000
Remaining amount of offsets to derivative net liability positions for hard and soft trigger provisions	$ 46,000,000