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, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Change in benefit obligations:
Benefit obligation at January 1	$ 143	$ 117
Service cost	10	9	$ 11
Interest cost	5	5	5
Actuarial loss (gain)	(14)	21
Benefits paid	(14)	(9)
Curtailments	(5)
Benefit obligation at December 31	125	143	117
Change in plan assets:
Fair value of plan assets at January 1	101	81
Actual return on plan assets	(8)	15
Employer contributions	12	14
Benefits paid	(14)	(9)
Fair value of plan assets at December 31	91	101	81
Funded status of plans at December 31	(34)	(42)
Other Postretirement Benefits [Member]
Change in benefit obligations:
Benefit obligation at January 1	17	13
Service cost	2	2	2
Interest cost	1		1
Actuarial loss (gain)		3
Benefits paid	(1)	(1)
Curtailments	(6)
Benefit obligation at December 31	13	17	$ 13
Change in plan assets:
Employer contributions	1	1
Benefits paid	(1)	(1)
Funded status of plans at December 31	$ (13)	$ (17)