Employee Benefit Plans (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets in defined benefit pension plan	$ 3,349	$ 2,949
Estimated employer contributions for defined benefit plans for the next year	$ 0
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	100.00%
Percent of qualified earnings contributed by employee matched by the Corporation	4.00%
Defined contribution plan expense	$ 24	22	$ 21
Qualified Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Estimated average life of pension plan in years	12
Fair value of plan assets in defined benefit pension plan	$ 3,350	2,933	2,458
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	$ 57	$ 56
Minimum | Qualified Plan | Debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	45.00%
Maximum | Qualified Plan | Debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	55.00%