Retirement and Employee Benefit Plans (Pension and Other Postretirement Benefit Costs) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Curtailment (gain) loss				$ (4)		$ 5
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost				10	$ 9	11
Interest cost				5	5	5
Expected return on plan assets				(7)	(6)	(6)
Amortization of net loss				2	2	2
Net periodic benefit cost				10	10	12
Curtailment (gain) loss		$ 1				1
Settlement loss						11
Total benefit cost (benefit)				10	10	24
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost				2	2	2
Interest cost				1		1
Net periodic benefit cost				3	2	3
Curtailment (gain) loss	$ (4)		$ (6)	(4)		(6)
Total benefit cost (benefit)				$ (1)	$ 2	$ (3)