Retirement and Employee Benefit Plans (Pension and Other Postretirement Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			$ 9	$ 11	$ 16
Interest cost			5	5	6
Expected return on plan assets			(6)	(6)	(9)
Amortization of transition obligation
Amortization of prior service cost
Amortization of net loss			2	2	2
Net periodic benefit cost			10	12	15
Curtailment loss	$ 1			1
Settlement loss				11
Net periodic benefit cost			10	24	15
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost			2	2	3
Interest cost				1	1
Expected return on plan assets
Amortization of transition obligation
Amortization of prior service cost
Amortization of net loss
Net periodic benefit cost			2	3	4
Curtailment loss		$ (6)		(6)
Settlement loss
Net periodic benefit cost			$ 2	$ (3)	$ 4