Employee Benefit Plans (Narrative) (Details) - USD ($)	1 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fixed earnings credit given to all new participants in pension and postretirement plans		3.00%
Percentage of bonds with highest yields excluded from calculation of yield curve for pension plan assumptions		10.00%
Percentage of bonds with lowest yields excluded from calculation of yield curve for pension plan assumptions		40.00%
Expected long-term return on plan assets		6.38%	6.75%	6.75%
Expected long-term return on plan assets in 2018		6.00%
Defined Contribution Plan, eligible compensation for deferral in abscense of election otherwise (percent)		4.00%
Defined Contribution Plan, employee benefit plan expense		$ 125,000,000	$ 122,000,000	$ 126,000,000
Qualified Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contribution	$ 200,000,000	$ 200,000,000	250,000,000
Pension plan, decline in discount rate (percent)		0.50%
Nonqualified Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contribution		$ 24,000,000	$ 31,000,000
Pension plan, decline in discount rate (percent)		0.50%