Retirement and Employee Benefit Plans (Narrative) (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
Defined Benefit Plan Disclosure [Line Items]
Change in accumulated other comprehensive income (loss)			$ (7)
Defined benefit plan included in accumulated other comprehensive income (loss), before tax			(42)	$ (31)
Defined benefit plan included in accumulated other comprehensive income (loss), after tax			(26)	(19)
Expected future net loss			(2)
Company's expected additional annual contribution			13
401 (k) Defined Contribution Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contribution expense			3	4	$ 3
Contributions capitalized			2	2	4
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Non-cash curtailment gain (loss)	$ (1)			(1)
Settlement loss				(11)
Lump-sum payment				37
Net periodic benefit cost (gain)			10	24	15
Curtailment gain (loss)	$ (1)			(1)
Employer contributions			14	10
Benefit obligation			$ 143	$ 117	138
Discount rate			3.75%	4.20%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Non-cash curtailment gain (loss)		$ 6		$ 6
Settlement loss
Net periodic benefit cost (gain)			2	(3)	4
Curtailment gain (loss)		$ 6		6
Employer contributions			1	1
Benefit obligation			$ 17	$ 13	$ 20
Discount rate			3.75%	4.20%