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, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair value of plan assets at January 1		$ 2,462,000,000
Fair value of plan assets at December 31		2,949,000,000	$ 2,462,000,000
Qualified Plan
Fair value of plan assets at January 1		2,458,000,000	2,747,000,000
Actual return on plan assets		579,000,000	(167,000,000)	$ 396,000,000
Employer contributions		0	0
Benefits paid		(104,000,000)	(122,000,000)
Fair value of plan assets at December 31		2,933,000,000	2,458,000,000	2,747,000,000
Projected benefit obligation at January 1		1,901,000,000	2,061,000,000
Service cost		31,000,000	29,000,000	29,000,000
Interest cost		80,000,000	75,000,000	78,000,000
Actuarial (gain) loss		223,000,000	(142,000,000)
Projected benefit obligation at December 31		2,131,000,000	1,901,000,000	2,061,000,000
Accumulated benefit obligation		2,121,000,000	1,893,000,000
Funded Status at December 31	[1],[2]	$ 802,000,000	$ 557,000,000
Weighted-average assumptions, discount rate, percent		3.43%	4.37%
Weighted-average assumptions, rate of compensation increase, percent		4.00%	4.00%
Accumulated other comprehensive income (loss), net actuarial loss		$ (463,000,000)	$ (687,000,000)
Accumulated other comprehensive income (loss), net prior service (cost) credit		121,000,000	140,000,000
Balance at December 31		(342,000,000)	(547,000,000)
Non-Qualified Plan
Fair value of plan assets at January 1		0
Benefits paid		(13,000,000)	(11,000,000)
Fair value of plan assets at December 31		0	0
Projected benefit obligation at January 1		211,000,000	212,000,000
Service cost		3,000,000	2,000,000	2,000,000
Interest cost		9,000,000	8,000,000	8,000,000
Actuarial (gain) loss		25,000,000	0
Projected benefit obligation at December 31		235,000,000	211,000,000	212,000,000
Accumulated benefit obligation		234,000,000	209,000,000
Funded Status at December 31	[1],[2]	$ (235,000,000)	$ (211,000,000)
Weighted-average assumptions, discount rate, percent		3.43%	4.37%
Weighted-average assumptions, rate of compensation increase, percent		4.00%	4.00%
Accumulated other comprehensive income (loss), net actuarial loss		$ (94,000,000)	$ (76,000,000)
Accumulated other comprehensive income (loss), net prior service (cost) credit		26,000,000	34,000,000
Balance at December 31		(68,000,000)	(42,000,000)
Postretirement Benefit Plan
Fair value of plan assets at January 1		56,000,000	60,000,000
Actual return on plan assets		5,000,000	(1,000,000)	2,000,000
Employer contributions		1,000,000	1,000,000
Benefits paid		(5,000,000)	(4,000,000)
Fair value of plan assets at December 31		57,000,000	56,000,000	60,000,000
Projected benefit obligation at January 1		46,000,000	51,000,000
Service cost		0	0
Interest cost		2,000,000	2,000,000	2,000,000
Actuarial (gain) loss		5,000,000	(3,000,000)
Projected benefit obligation at December 31		48,000,000	46,000,000	$ 51,000,000
Accumulated benefit obligation		48,000,000	46,000,000
Funded Status at December 31	[1],[2]	$ 9,000,000	$ 10,000,000
Weighted-average assumptions, discount rate, percent		3.26%	4.26%
Healthcare cost trend rate assumed for next year		6.25%	6.50%
Rate to which the healthcare cost trend rate is assumed to decline (the ultimate trend rate), percent		4.50%	4.50%	4.50%
Year when healthcare cost trend rate reaches the ultimate trend rate		2027	2027
Accumulated other comprehensive income (loss), net actuarial loss		$ (20,000,000)	$ (19,000,000)
Accumulated other comprehensive income (loss), net prior service (cost) credit		1,000,000	1,000,000
Balance at December 31		$ (19,000,000)	$ (18,000,000)

[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.