Employee Benefit Plans (Reconciliations of Plan assets and the Projected Benefit Obligation, the Weighted-Average Assumptions Used to Determine Year-End Benefit Obligations, and the Amounts Recognized in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair value of plan assets at January 1		$ 2,448,000,000
Fair value of plan assets at December 31		2,743,000,000	$ 2,448,000,000
Qualified Plan
Fair value of plan assets at January 1		2,453,000,000	2,346,000,000
Actual return on plan assets		396,000,000	200,000,000	$ (73,000,000)
Employer contributions		0	0
Benefits paid		102,000,000	93,000,000
Fair value of plan assets at December 31		2,747,000,000	2,453,000,000	2,346,000,000
Projected benefit obligation at January 1		1,902,000,000	1,916,000,000
Service cost		29,000,000	31,000,000	35,000,000
Interest cost		78,000,000	87,000,000	88,000,000
Actuarial (gain) loss		154,000,000	161,000,000
Plan amendment		0	200,000,000
Projected benefit obligation at December 31		2,061,000,000	1,902,000,000	1,916,000,000
Accumulated benefit obligation		2,052,000,000	1,894,000,000
Funded Status at December 31	[1],[2]	$ 686,000,000	$ 551,000,000
Weighted-average assumptions, discount rate, percent		3.74%	4.23%
Weighted-average assumptions, rate of compensation increase, percent		3.75%	3.50%
Accumulated other comprehensive income (loss), net actuarial loss		$ (548,000,000)	$ (673,000,000)
Accumulated other comprehensive income (loss), net prior service (cost) credit		159,000,000	178,000,000
Balance at December 31		(389,000,000)	(495,000,000)
Non-Qualified Plan
Fair value of plan assets at January 1		0
Benefits paid		11,000,000	11,000,000
Fair value of plan assets at December 31		0	0
Projected benefit obligation at January 1		201,000,000	222,000,000
Service cost		2,000,000	3,000,000	4,000,000
Interest cost		8,000,000	10,000,000	10,000,000
Actuarial (gain) loss		12,000,000	11,000,000
Plan amendment		0	34,000,000
Projected benefit obligation at December 31		212,000,000	201,000,000	222,000,000
Accumulated benefit obligation		209,000,000	198,000,000
Funded Status at December 31	[1],[2]	$ (212,000,000)	$ (201,000,000)
Weighted-average assumptions, discount rate, percent		3.74%	4.23%
Weighted-average assumptions, rate of compensation increase, percent		3.75%	3.50%
Accumulated other comprehensive income (loss), net actuarial loss		$ (85,000,000)	$ (82,000,000)
Accumulated other comprehensive income (loss), net prior service (cost) credit		42,000,000	50,000,000
Balance at December 31		(43,000,000)	(32,000,000)
Postretirement Benefit Plan
Fair value of plan assets at January 1		62,000,000	61,000,000
Actual return on plan assets		2,000,000	2,000,000	0
Employer contributions		1,000,000	4,000,000
Benefits paid		5,000,000	5,000,000
Fair value of plan assets at December 31		60,000,000	62,000,000	61,000,000
Projected benefit obligation at January 1		55,000,000	59,000,000
Service cost		0	0
Interest cost		2,000,000	3,000,000	3,000,000
Actuarial (gain) loss		(1,000,000)	(2,000,000)
Plan amendment		0	0
Projected benefit obligation at December 31		51,000,000	55,000,000	$ 59,000,000
Accumulated benefit obligation		51,000,000	55,000,000
Funded Status at December 31	[1],[2]	$ 9,000,000	$ 7,000,000
Weighted-average assumptions, discount rate, percent		3.55%	3.92%
Healthcare cost trend rate assumed for next year		6.50%	6.50%
Rate to which the healthcare cost trend rate is assumed to decline (the ultimate trend rate), percent		4.50%	5.00%	5.00%
Year when healthcare cost trend rate reaches the ultimate trend rate		2027	2027
Accumulated other comprehensive income (loss), net actuarial loss		$ (19,000,000)	$ (20,000,000)
Accumulated other comprehensive income (loss), net prior service (cost) credit		1,000,000	1,000,000
Balance at December 31		$ (18,000,000)	$ (19,000,000)

[1] Based on projected benefit obligation for defined benefit pension plans and accumulated benefit obligation for postretirement benefit plan.

[2] The Corporation recognizes the overfunded and underfunded status of the plans in “accrued income and other assets” and “accrued expenses and other liabilities,” respectively, on the consolidated balance sheets.