Retirement and Employee Benefit Plans (Changes in Plans Benefit Obligations, Fair Value of Assets, and Funded Status) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Change in benefit obligations:
Benefit obligation at January 1	$ 126	$ 125
Service cost	7	7	$ 10
Interest cost	5	5	5
Participant contributions	0	0
Actuarial loss	16	15
Benefits paid	(13)	(2)
Plan amendments	0	0
Curtailments	(2)	0
Settlements	0	(24)
Benefit obligation at December 31	139	126	125
Change in plan assets:
Fair value of plan assets at January 1	96	91
Actual return on plan assets	11	16
Employer contributions	12	12
Participant contributions	0	0
Benefits paid	(13)	(2)
Settlements	0	(21)
Fair value of plan assets at December 31	106	96	91
Funded status of plans at December 31	(33)	(30)
Other Postretirement Benefits
Change in benefit obligations:
Benefit obligation at January 1	13	13
Service cost	2	1	2
Interest cost	0	0	1
Participant contributions	0	0
Actuarial loss	1	1
Benefits paid	(1)	(2)
Plan amendments	(2)	0
Curtailments	0	0
Settlements	0	0
Benefit obligation at December 31	13	13	13
Change in plan assets:
Fair value of plan assets at January 1	0	0
Actual return on plan assets	0	0
Employer contributions	1	2
Participant contributions	0	0
Benefits paid	(1)	(2)
Settlements	0	0
Fair value of plan assets at December 31	0	0	$ 0
Funded status of plans at December 31	$ (13)	$ (13)