Employee Benefit Plans (Narrative) (Details)	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets in defined benefit pension plan	$ 2,949,000,000	$ 2,462,000,000
Effect of one percentage point increase on postretirement benefit obligation	0.05
Estimated employer contributions for defined benefit plans in year 2017	$ 0
Employer core matching cash contribution as percent of employee contribution	100.00%
Percent of qualified earnings contributed by employee matched by the Corporation	4.00%
Defined contribution plan expense	$ 22,000,000	21,000,000	$ 21,000,000
Profit sharing plan expense included in employee benefits expense	0	0	1,000,000
Qualified Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets in defined benefit pension plan	$ 2,933,000,000	2,458,000,000	2,747,000,000
Estimated average life of pension plan in years	12
Non-Qualified Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets in defined benefit pension plan	$ 0	0
Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets in defined benefit pension plan	$ 57,000,000	$ 56,000,000	$ 60,000,000
Minimum | Qualified Plan | Debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation percent	0.45
Maximum | Qualified Plan | Debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation percent	0.55