Retirement and Employee Benefit Plans (Narrative) (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]
Non-cash curtailment gain (loss)				$ 4		$ (5)
Change in accumulated other comprehensive income (loss)				6
Defined benefit plan included in accumulated other comprehensive (income) loss, before tax				34	$ 42
Defined benefit plan included in accumulated other comprehensive (income) loss after tax				20	26
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	3	4
Contributions capitalized				2	2	2
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Non-cash curtailment gain (loss)		$ (1)				(1)
Settlement loss						(11)
Lump-sum payment						37
Employer contributions				12	14
Benefit obligation				125	143	117
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Non-cash curtailment gain (loss)	$ 4		$ 6	4		6
Employer contributions				1	1
Benefit obligation				$ 13	$ 17	$ 13