Employee Benefit Plans (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit pension expense	$ 6,000,000	$ 47,000,000	$ 39,000,000
Postretirement benefit expense	6,000,000	48,000,000	40,000,000
Fair value of plan assets in defined benefit pension plan	2,448,000,000	2,347,000,000
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	22,000,000	22,000,000
Profit sharing plan annual contribution to the individual account of each eligible employee, minimum percent of annual compensation	3.00%
Profit sharing plan annual contribution to the individual account of each eligible employee, maximum percent of annual compensation	8.00%
Profit sharing plan expense included in employee benefits expense	$ 10,000,000	10,000,000	10,000,000
Qualified Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Postretirement benefit expense	$ (9,000,000)	27,000,000	23,000,000
Estimated average life of pension plan in years	12
Fair value of plan assets in defined benefit pension plan	$ 2,453,000,000	2,346,000,000	2,541,000,000
Qualified Plan | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation, minimum percent	45.00%
Target allocation, maximum percent	55.00%
Qualified Plan | Debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation, minimum percent	45.00%
Target allocation, maximum percent	55.00%
Non-Qualified Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Postretirement benefit expense	$ 15,000,000	20,000,000	16,000,000
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]
Postretirement benefit expense	0	1,000,000	1,000,000
Fair value of plan assets in defined benefit pension plan	$ 62,000,000	$ 61,000,000	$ 67,000,000